Well I just took the whole thing apart and noticed that on at least Vram there was no mark on the heatsink from the thermal pads. So I think that either they are too thin or that the heatsink wasn't seated correctly. It has 4 spring bolts and two were jammed, so that they didn't push in and out against the spring. Question is, if the Vram chips were overheating, would the iMac put the CPU into some sort of idle mode? I’ve ordered some Artic T-3 0.5mm,1mm and 1.5mm thermal pads (100mm by 20mm x4). Hopefully, that will fix the issue and I haven’t damaged the VRAM chips. If it is still not working then I will re-fit the i5 cpu. I did run Etrecheck but that just said the system was slow and show up any hardware issues? Cheers john