Turning off Black Bar Detection makes SPV behave strangely.

So I am using SVP with RIFE 4.4 (so far only 4.4 can handle big 4k movies), I'm on an AMD card (7900XTX) with driver 25.5.1, trying to watch HUGE 4k movies.  It works fine most of the time, except recently I had some trouble and had to switch to Vaporsynth and MPC video renderer.  Anyway, now it has this weird behavior if Black Box detection is OFF.  The first screenshot is with BBD on, the second is with it off (having it off causes stuttering every 3 to 4 seconds with smooth periods between).  Only problem is, I don't want it on, because if a movie changes aspect ratios in one scene, it can cause glitchy behavior (a second of black screen, or causing the next scene to be in a wrong aspect ratio for a few seconds).  Any idea for a solution?