my cadillac is running hot only when it sits still with the heat on

When a car is sitting, the revolutions of the engine are lower. The water pump is circulating less coolant through the radiator. The radiator has less cooling air since the car is not moving.

I agree with Tenspeed. Perhaps you can tell us what you have done thus far to cool down the engine. For example have you checked the anti freeze level, are there any leaks, checked the oil, or checked for water in the oil, is the fan belt tight...point is what cause and effect have you done?

if a northstar engine,check the pump drive on rear of bank 1 tension may be off small belt,

check ur cooling fans the sensors often go bad causing fan to only run with ac fan pulls air through radiator when sitting still dont need them when driving because the air is flowing plus water pump turns faster