sensor, sinchro, and now head gasket!

My check engine light came on last April and soon after that my van began to hesitate (mostly when idling). The first garage I took it to said their computers showed it was the camshaft position sensor. They replaced it, but the light stayed on. I took it to Ford, and they said it was the camshaft syncho. They replaced it, but the light stayed on. Then they told me they would need to replace some sort of module. After the module was replaced, the light went off and I passed the emission check. Unfortunately, the van did not stop hesitating. This hesitation has gotten worse and my light is back on again. Ford is now telling me that is a bad head gasket and that my plugs need changing. They want $1800 from me. I've already paid them $1000 and my problems have not been solved. How can I tell if the headgasket is really the problem?

Depending on where the head gasket is bad you may have air in the coolant, oil in the coolant or coolant in the oil. You could also see significant water vapour in the exhaust. Removing a spark plug may reveal information too.

A compression test may also help to verify. Ask them how they came to the conclusion that a head gasket is bad.

Have you ever been given the actual code numbers?
What module was replaced?
You could have a stuck EGR valve too. (That would cause a rough idle.)