1995 mustang 5.0 overheats when I get down on it , have changed water pump, thermastat and checked for air pocket and did a head gasket check and nothing , any ideas ?

 

possibly the radiator is a bit cloggy. fan working?

 

Yeah fan cuts on , and I flushed the radiator , but I'm thinking its the serpentine belt , cause when I bought it it wasn't connected to the smog , so they re rigged it and I think the have the water pump going backwards ,

 

Lol, that will do it. Good catch.

 

is the groove side of the belt on the water pump pulley?

 

as long as the flat side of the belt is on the pump its turning the correct direction.

 

Still stumped , checked and the waterpump is smooth and going the correct way , I have no clue , last thing i can think of is maybe the gauge is wrong and reading to hot , but I doubt it

 

If the temp immediately comes back down after you're not "on it" anymore, its flow related.

 

It's not flow then because it takes around 20 mins for it to cool back down

 

Then your description is faulty. Each piece of the cooling system has it's own individual symptoms when they fail. Properly describing the way your car overheats is the only way to get help online. It doesn't make sense if it only overheats as a reaction to spirited driving, and then takes 20 minutes to return to normal.