zadam123,
I think I know what your problem is. You've described exactly what my VX was doing for about 2 years and it was very frustrating.
Turned out one of the head gasket bolts had broken (the one closest to the front on the passenger side). When the engine would get hot enough, you get a leak between the combustion chamber and the coolant. It's pretty much a one way leak that results in the combustion chamber pressurizing the coolant and screwing up the flow. I say it's only a one way leak because I wasn't losing any coolant, which makes sense since the pressure in combustion chamber should be much higher than that of the coolant.