facts:
- temp starts to go up after sitting idle for 5-10 minutes
- rad fan comes on at correct times
- even with fan on, temp still goes up
- giving it some throttle in neutral/park causes temp to go back down
- when in motion, temp is no problem
- radiator was replaced 45k-50k miles ago (due to collision)
- I have plenty of rad fluid, none of it has leaked out

I know its not the temp sensor (fan switch) because unplugging it causes the fan to turn off. plug it back in, fans kicks back on.

so thermostat then? or would a coolent flush solve the problems?