1998 GMC Savana 2500 5.7 Lt. (idle issues)

[Fecklar]

Hi Everyone,
I have a problem that nobody seems to have an answer for, so I thought I would give this a try. I have a 1998 GMC Savana cargo van. It intermittently has a high idle problem. Upon startup, it idles really high (about 2500 rpm) for about 7 seconds and then gradually goes down to almost normal. This happens only sometimes and even when the engine is hot. Always happens when it's cold. Even when you are driving with your foot off the gas, the engine will shift from 1st to second gear by itself. If you are coasting with your foot off the gas, it will stay the same speed for about 30 seconds before it slowly begins to slow down. I checked the accelerator cable and it is totally free moving. The engine had a complete tune up about 15,000 kms ago. There are no codes present. I took it to the dealer 3 times. The first time they cleaned the throttle body and inspected the cap and rotor. They also relearned the IAC. This seemed to solve the problem for a couple of days. I took it back to the dealer and they changed the TPS switch. This made no difference. The third time they said wait until it does more often and bring it back. Any suggestions would be appreciated. The vehicle is a 5.7 litre engine with 861,000 original Kms.
Thanks in Advance