Sounds like a PSU-heat related voltage drift. The 4.5A PSU is a good one in terms of regulation, but will require a re-cap after all these years. Please have that checked.
I tried with another PSU and same problem. Anyway, I'll recap both because they are old A500 PSUs.
I tested putting my A1200 with the new SuperPlus resolution and for now, I have zero problems. The issue appears only using HighGFX.