High memory usage with dynamic subtitles

Well, this is embarrassing. I have no idea. Overall, video decoding isn't my specialty, I usually fiddle with player settings until I get it working as I want to.

If I understood Media Player Classic's tooltip's correctly, I'm using its internal subtitle renderer, and EVR Custom Pres. for Direct Show video.
I think however, that the MemoryLeaksDetails mod.zip fixed the issue, as I played both the proportionally biggest video I have (1.22Gb for 24 min! 1080p + 3 audio streams, 2 of them in flac lol) and a 51 min 720p video with no issues. Both had ASS (talk about an unlucky acronym) lossless compression subtitles.
Memory usage for Media Player Classic didn't even reach 600 Mb when playing the 1080p video @ 60fps.

P.S.: I should note that I've set all 4 profiles to the same settings.

Well, just for the sake of science, I watched another episode of that particular anime where I experienced the "memory leak" (if it's fit to call it so).
I got a sign at the same spot where I got the memory leak ones before, I took a screenshot of it but I was so dumb of copying some text before pasting it on an image editor.
If my memory serves me right, it was something about an error when generating an errorlog (oh, the irony).
Afterwards I noticed the ending on that anime is rather merciless on motion interpolation. Here's a link to said ED (Youtube link). Up to 0:57, it's like it was designed to not be "SVP'd". They'll have to try harder >:3
SVP takes it like a boss and generates not only the frames needed for 60 fps but also no artifacts (well, no artifacts that I could see)
Then I did a quick test on that ending and watched it (on 720p) with the performance graphs running.

Then, I did the same but on fullscreen (I have a 1080p display). That's when MPC's memory usage hit over 1100Mb, and in less than 1 minute! It peaks specifically on the part with a girl with a black dress. It's like a crazy motion interpolation test   

The 400-something Mb is me being a fool and not noticing MPC continues to use memory while on pause.
I think I should note that while I wasn't exactly noticeable due to a lack of a clear movement pattern, the first half of the video had very unstable framerates (even below 24) according to Fraps.

After some testing, I have determined it's the particular encoding of some particular episodes, plus my 1.5Tb Seagate hard drive (ST31500341AS) acting up as I moved the videos to my other drive, a 640Gb WD (WD6400AAKS-00E4A0) and things performed slightly... less bad.

Putting it on clearer terms: I have this video sequence https://www.youtube.com/watch?v=tUS7gSv2ZxE
On "encoding A" and "encoding B".
Encoding A performs excellently on both hard drives.
Encoding B has very poor performance on Seagate hard drive, slightly less bad on WD hard drive.

My internet connection isn not exactly fast for uploads, so uploading a ~300Mb video file for the final 1:30 minutes isn not exactly profitable, and if I re-encoded the troublesome bit and sent it I would lose the particular encoding that triggers this issue.
As I obtained the video file on a "t0rr3nt" site, I'm not entirely sure if I'm allowed to post a such a link here, in addition to the fact I have yet to find a "Forum Rules" section xP
I would PM you but I have found no way of doing so either so... what to do?

I will try to upload the video later today, though.

I just went ahead and uploaded it while I was out for a while.
http://filesmelt.com/dl/Motion_interpolation_test.mkv
It's a whole episode. I didn't check closely but apparently the whole thing plays ok, with the exception of the ending credits (starting around 22:20) that refuse to play at 60 fps.

Thanks
For some reason I didn't even consider subtitles as a cause for the problem, so I didn't disable them to confirm. I will edit the first post title to more accurately tell what causes this issue.

Edit:
For science! I tried changing the renderer in MPC output settings as well as subtitles settings. I got some interesting results.
I quickly found out that I could get stable framerate (almost, it dropped to 59, 58 fps a couple times) on these troublesome dynamic subtitles by using these settings:

Instead of the default ones:

The texture size at 720p did not represent a noticeable loss on fullscreen (and I have a 1080p display).
I could not find a fix to the high memory usage, but I did notice that different output renderers had slightly different memory performance. I did not make enough runs to call this a truly reliable "scientific" test, but (always on a 720p window):
-VMR-9 (renderless) peaks to about 900 Mb.
-EVR Custom Presenter peaks barely over 900 Mb, and not always gets over 900.
-Haali has the lowest memory usage of all, doesn't reach 600 Mb, but it also had flashing subtitles on my settings
-madVR seems to hit a "limit" at 910 Mb, but always reaches that level.

Something noteworthy: Disabling "Allow animation when buffering" on MPC Subtitles options allows to free up memory much quicker.
I recalled one of these had a higher memory usage so I played the ending with all renderers again but with subtitle textures set to 1920x1080. It would seem Haali uses a lot more memory (even when the window size was 1280x720) with higher subtitle textures. I tried getting to an optimal setting with the Haali renderer with no avail, as it requires "Allow animation when buffering" enabled so subtitles are constantly visible.

Using EVR Custom Presenter and the subtitle settings I posted before I got to this performance graph and a stable framerate all along (on a 1280x720 window). Maximum memory usage was 906 Mb.

Yes, I played the same bit of video like 20 times in a row, that music is going to resonate in my head for days.

If you're not going to be rendering subtitles at 1920x1080, there's no point in using MPC-HC's internal subtitle renderer. xy-VSFilter is significantly faster and better.