Hi Scotty1, if the thermostat doesn't fix the problem, rev the engine to about 1700 RPM and look at the lower rad hose. If the lower rad hose is being sucked flat then your rad requires steam cleaning.
Had your problem on 454 and rad was all scaled up and a good steam cleaning removed all the scale and let the water circulate properly. The hot water was being pumped out of the engine to the top of the rad but it could not flow down thru the rad tubes because they were clodded with scale.
Hope this helps