I can't help with the glitches on NTSC unless there is some issue with your A1200? The Indivision only displays what it gets from the A1200.
As for HighGFX it's the same on most A1200's (even with Indi MkI), that screen size is just pushing the hardware too far, you can try with only 16 colours you might get better results but I suggest using 'SuperPlus' driver 800x600, should be glitch free and is still a nice size screen, works well with 64 colours with plenty of chip Ram left.
