i've been having this problem for close to a year now. i have a 305 v8 and whenever my car sits for 5 minutes at running temp it will then start to heat up and run bad. my fluid level is good and the thermostat was just changed 2 months ago.

 

Check to make sure your radiator fan is turning on. If not, that's the problem, but you need to find the cause.

 

the fan is turning on... i think it may be the water pump but im not sure if thats what is gonna be the issue

 

what is the temp? these cars were run hot from the factory. 235 is fairly normal

 

oh ok its been runnin 235-240 when it heats up

 

That's kinda toasty, my 91 never got near that temp.
Have you checked your ignition timing? If it's off (especially retarded) it can make the engine run hotter.

 

Running Temperature on my 89 is 220 F. In winter it never gets over 160 really, and I have a thermostat that opens completely at 175. It used to overheat bad, but a mix of a new therm. and changing it so that the fans come on at 160 also helps keep it at 220-230 when I'm sitting in traffic on a hot Phoenix Summer day.

My IROC is also a 305. Check to see at what temp the fans come on. Also, I did replace my water pump, though it wasn't bad. I had the top end of the motor off and took the safe route to replace it.