F-350 Transmission: odd behavior?

It has only happened on two occasions recently. The transmission does not up shift properly and the truck will not get over about 30 mph. Acts and sounds like a governor does. I can push the gas to the floor and the transmission seems to stall. If I let off the gas a little and then reapply the gas it will sometimes shift and gradually gain speed. I have had the transmission fluid and filter changed between both occurrences. The only commonality I can find is "Heat". The first time it happened I had been in Atlanta stop & go traffic for about 1.5 hours and it was toward the end of the drive that it happened. The second time was yesterday. Again it occurred near the end of my drive, but I had been sitting idle for about 45 minutes earlier (It was shut off for about 20 minutes after the 45 minute sitting idle). Someone told me it might be a solenoid sticking. The mechanics can't diagnose it because it does not do it when I take it to them.
Any thoughts?
'99 F-350 Super duty V10 4x4 dually long bed

have a shop or even a quick-lube flush it... a pan drop only gets half the cruddy fluid and none of the dirt out of the solenoid pack...

try that first.

sound like the pressure releaf valve is sticking try to flush it first with some BG tranny flush if that doesnt work it probally needs avavle body (ex ford tranny builder)