[John Deere]

Yet another update - brought the car to a proper mechanic and not those yahoos at the GM dealership - he did a block test right in front of me...C02 detected - slow head gasket leak causing an air lock around the temp sensor from expelled compression. New head gasket being installed now.

A lesson to all you car guys out there - you know your car, and if you know something isn't right, don't let some tech tell you otherwise - find a good mechanic - he's worth his weight in gold!

I'll let you know if this solves everything.

Thanks to all who replied with great suggestions.