Hello to all! I am a fairly new GTO Owner. I got hold of a manual MK1,1992 GTO twin turbo at 70k miles in quite a bad physical shape due to being left to rust for over 5 years. I started doing some cosmetic work to bring it back to its glory.
The timing somehow slipped and replaced belt and water pump. Lucky I had no damage to valves and it runs great. I still have a very whiny power steering and a slight overheating problem.
Try flushing the system as if its been stood for that long you might have a slight blockage somewhere. Did you do the thermostat the same time you did your belt?
The whiney power steering could well be your pump. Drain and chuck the oil, replace pump and new oil, turn from lock to lock to bleed the system and it should be ok. Was the same on my Mrs. car (not a GTO tho).