does it run bad before stalling and how does it start up after sitting?
After it has been sitting it starts normal and runs fine for about 5-10 minutes, then it starts sputtering and eventually stalls.
There is a bulletin for a new design upper intake manifold. The old plastic one had the egr passage next to a coolant chamber. It would get too hot and melt and coolant would get sucked into the passeges for air/fuel delivery. This could cause the engine to hydro lock if bad enough. Check your coolant level. Other than knowing the intake has been replaced the only way to know for sure this is not the issue you would have to remove the intake. You may also have a catalytic converter clogging, if you pull the front 02 sensors and it doesn't stall you found the problem. This would be covered under the federal emissions warranty 8/80. This is a known issue also. Lets start here.
It is possible,I should have said check that first. I have seen much more clogged cats. Intakes more on the older models.
You can verify that buy checking fuel pressure(53-39 psi key on engine off). If you have not changed the fuel filter its a good place to start.