I'm beginning to think the frame drops may only be specific to a few games. I did some more testing and here are my findings:
Nothing in the TV settings changes the behavior. I checked way more things than can be listed here.
General MiSTer settings do not affect the frame drops. Resolution, color depth, vsync_adjust 0,1,2 change nothing.
Focusing on only NES and SNES, I tried the Test Suite ROMS and a few games.
NES
- 240p Test Suite (PD) v0.21 pinobatch.nes
All scrolling tests are perfectly smooth, vertically and horizontally at every speed.
Drop shadow test is perfect.
- Metroid - Start of new game, run right to door, then left repeatedly. Frame skip happens in the exact same places, regardless of timing. Did not notice that pattern before. Tested with and without extra sprites.
- Final Fantasy 1 - no frame drops anywhere walking in towns, with or without b-button dash.
- Super Mario Brothers - no frame drops noticed in first stage, walking or running.
SNES
- 240p Test Suite (PD) Artemio v1.03.sfc
All scrolling tests are perfectly smooth, vertically and horizontally at every speed.
Drop shadow test is perfect.
- Super Mario World - Stage 1 (to the right of Yoshi's house on world map) - walking to the right shows no frame drops, but running to the right and left shows frame drops. They seem to be in the same general areas, but not exactly like Metroid. Tested with and without turbo CPU.
- Final Fantasy II and III - Intros have lots of vertical scrolling and some horizondal. No frame drops.
I'll connect the MiSTer to my old plasma later to see if the frame drops in Metroid are happening there and I just didn't notice it before. That was the first game I fired up right after getting this new LG C1 OLED, and I noticed the frame skipping right away. Maybe just coincidence combined with OCD? I'll also try some other games and see if there are more that show frame skipping.
And I didn't notice any frame skipping in Neo-Geo or Genesis. I tried Samurai Showdown IV scrolling intro text and Sonic background scrolling.