[Dessertrunner]

I have had two SVX's do head gaskets and both when tested were said to be okay. Yet it was found both had headgaskets gone. You need to understand a couple of things about the svx cooling system.

-- The heater is always on its just that when you turn it on at the dash it blows air past the evaporator.
-- Its possiable for the heat gasket to cause the coolent to boil into the overflow tank but when the engine cools down it sucks the coolent back so when you later check its still full.
-- The fans only go on to low if the AC is off unless you are with in a certain speed range.
-- Let car idle with the AC off and the fans should turn on and off as the engine passes 95C temp. If this doesn't happen your problem is fans.

The other reason the car overheats on long trips and not short is that you could have a leak from the compression chamber and this pressure builds over time in the radiator.
One suggestion I have is get the car hot after driving for say 50 miles then use a rag and remove the radiator cap. The coolent may boil as the pressure drops and if you top it up (with the engine running) then with radiator cap off rev the engine up. If its a head gasket it blows the water out. Depending on how bad the leak is the coolent will jump 6in above radiator. Hope that helps
Tony