I hardly ever use GR mode, but happened to end up in it after breaking out of Alphaplus, and noticed that if you print text that makes the screen scroll, it will make the screen vertically shift.

100 GR:FOR I=0 TO 39:R=RND(1)*16:COLOR=R:HLIN 0,39 AT I:NEXT
110 PRINT : GOTO 110

the screen will "wiggle". It's actually kind of a neat effect.

Interestingly, whenever it wiggles, it causes a partial screen update message to display if I turn on the frameskip osd with F11.

If you vary the frameskip, you can see the numbers change on the partial screen update message. If I set frameskip to 10/10 the partial screen update numbers slow down where you can actually read them.

Also, if you put an HGR in the above program, like 105 HGR the screen glitches show up. It's interesting that the CALL -912 routine at FC70 makes some hits to softswitches.

Printing text in the print statement shows that the text window part doesn't wiggle.

If I change line 110 to 110 CALL -912 : GOTO 110 it will do the same thing.

[Linked Image from i.imgur.com]
Putting them side by side, you can see the bars have shifted vertically:
[Linked Image from i.imgur.com][Linked Image from i.imgur.com]
adding "105 HGR" to the program:
[Linked Image from i.imgur.com]

The text part is solid, it doesn't wiggle at all:
[Linked Image from i.imgur.com]
Maybe that can give some clues as to what's happening.