When this car is warmed up the heater blows cold until I rev the engine it will immediately turn warm for a short time until I rev again. Any Ideas? Will it be hard to fix or expensive?

It sounds like the auxiliary water pump is gone.

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1999 Porsche 996 Carrera Convertible
1994 420E - SOLD
1986 300E - SOLD, what a car
609 Certified

Sounds as if it is low on coolant.

Good luck,