Have had this happen to me in the past. It could be the following:
- 1D4 revision of the mainboard. These do require a Timing fix. E123c or r and E125c or r to be removed. (rev 2B require this as well)
- GAYLE chip revision
- Power Supply
- Board may need recapping if not already done.
This is why when running A1200’s I try and use the 1D1 revision. As I personally have had better results.
Cue the Sheldon Coopers for the rebuttal on this 🙄