Bob - I hate to say it, but sounds like you have the same problem I had on Rhapsody with clogged cooling passages. Is your engine raw water cooled? If so, check out the description of the symptoms on my 2GM rebuild.

I now make sure that before the engine is shut down that it is run at about 1200 rpm with no load for 5 minutes to cool down the block and prevent salts & scale from depositing in the cooling passages due to boiling water.