My indivision AGA mk2 had the same artefacting due to heat, Lisa went insanely hot and within minutes everything went haywire, a good polygon based game to stresstest with was 4D sports driving (aka stunts) running that in whdload kicked me to workbench with unrelated error codes quick when cooling was insufficient
So as others have said try just to run the miggy case open and maybe some kind of fan moving some air, mine needed a fan directly on the indivision/Lisa to function.
This is how I fixed it:
