1990 Corvette TPI, overheating and high idle.

I am baffled at the moment. I Just picked up this 1990 Corvette with the L98 TPI. The motor was replaced with a Jasper Class 1 performance engine 7,000 miles ago. Now, I had some problems with the cars idle. I was told it may have been the IAC valve so I replaced that. Only for it to know make a horrible hissing noise which means a vacuum leak, I never reset the valve properly since I do not know how!

But the worse of problems is the overheating. Starting 2 weeks ago, whenever I am driving the car for a at least 30 minutes, the car will start to heat up while sitting at idle. If I have the AC on, the car will not heat up as bad. I have noticed the car is able to heat up to almost 260*F whereas normal temp is 190*F- 210*F. Oil Temp will get to almost 210*F at most. I just changed the Thermostat to a 160* stat, also refilled the system with 50/50 green antifreeze. I bleed the enitre system out, and noticed water flow. However, at times I do hear the fan will not kick in, then sometimes it will at the proper time. I am not leaking any fluids, and the previous owner allowed the car to sit a majority of the time. I just changed the oil, 10w-30. It is still a factory car outside of the motor and the T-stat.

to add: If I have the AC on from start-up to shut down, the car will stay at the 160* stat's operating temp. If I have it off, the car will be ok as long as I am consistantly moving. But if I am idleing or slowly moving in traffic, it will heat up and heat up quick!

Please help! I traded a perfect trans am for this piece! I am hoping its just a relay problem!

thanks Ryan