Runnig Hot!!!

The van began running hot, but would cool down. Took in to a local MIDAS Shop and the lower intake manifold gasket was shot. Got that replaced and the Mechanic said it is still running hot and he suggested replacing the thermostat. Got that replaced. Mechanic said your good to go. Well I got into and guess what it still ran hot. Took back and they sent me to another MIDAS Shop and had the sensor right at the thermostat replaced, because it was running hot the the fans were not kicking on. Then checked the bleed off valve on the return line and it was squirting out fluid, then checked the bleed off valve right above the water pump and it squirted out fuild but then tappered off to nothing. The mechanic said the water pump needed replacing, so I did that myself and I''ll be damned if it still isn't running hot. It only runs hot for just a bit and I'll shut it down let it cool and it runs like a champ. Why is running hot at all with everything that has been done?
The vehicle is a 1997 Pontiac Transport with 191,000 miles. Got it when it had 88,000 miles and no problems till now.

I think you are blessed with the classic 3.1/3.4 blown head gasket syndrome. Sorry. I'm surprised experienced mechanics did not hint from the start on this very well known flaw. THere are hundreds of such threads across Transport, Lumina APV, Venture and Montana forums, just to mention the vans. (search with key words "blown head gasket"). Sorry for the futility of these changes: pump, thermostat but at least, you saved the radiator, something others had tried too, to no avail.....

Thanks for the input. They even told me at the shop that the GM 3400 series motors where know for the manifold gaskets blowing, just wonder why they didn't say anything about the head gasket. Looks like I got my work cut out for me on replacing those head gaskets. Guess it was be leaking water around one of the water ports near the piston is why I don't have water in my oil and vice versa. Again thanks, I was just hoping it would be something easier to replace.