302Z28

I have been having problems with a rough idle and a giant flat spot when accelerating from idle. This only seems to happen when the engine is at full operating temperature. No problem when it is cold. Four different shops (including the local Chevy dealer) have looked at it. The following parts have been changed, but have not solved the problem: plugs, plug wires, coils, TPS, air filter, fuel filter, fuel regulator, injectors, IAC, MAP, 02 sensor, EGR, head gasket, head has been crack checked, motor mounts. Exhaust system has been checked for blockage. The car is now very hard starting, idles very rough and seems to have an over rich condition. Occasionally it will stall when pulling away from a stop. High speed operation is fine. For some time now, I have noticed a small puddle of antifreeze on the left side of the transmission bell housing. I was unable to locate any leak. Yesterday, I removed the plug from the temp sender and a small amount of antifreeze came out. Could this sender be part of the problem i'm having? The temp gage still works fine. Could the IAT sensor be part of the problem?
Thanks for any help