Need help with two GP's with same problem running.

Ok here's the problem. I have a 2003 grand prix se 3.1 and a 1998 grand prix gtp series II sc. Both have the exact same problem running and both had overheating issues prior to. I can start either one on a given day while cold and drive them as if nothing is wrong. But soon as either one hits operating temp, I start to hear what sounds like a rattling or sputtering like sound from under the hood, accompanied with a sudden loss of power and then the engine shuts off. Now yesterday I attempted further diagnostics with the se model GP. I let it idle rather than driving it this time, and it ran perfectly fine for about 20 mins, started to make the aforementioned noise, I revved the engine to about 2000 rpm and it kept it from cutting off and smoothed, but the second I let up, it was off and wouldn't restart till it was cooled off. Same goes for the gtp. I'm really stumped on this one.

Sure this rattling sound is not coming from your converter? If your converter is partially blocked, it can cause a stall/hard restart.....take a rubber mallet and hit the converter to hear a possible rattle....

Two different engines.....each has their own faults...

Have they been scanned for codes?

Do both restart when they die?

Do they lack power after a restart?

Losing fluids(levels should be checked on cold engines).....

I used my scan tool on both, no codes pointing to the issue. The gtp is reading a multiple misfire code, the SE is reading a small evap leak code. They won't start immediately after cutting off. Usually I have to wait till cooled off before being able to restart either. With them being completely different drivetrains, it's just odd to me for them to have identical symptoms. Granted the gtp has the misfire as opposed to the SE. I checked spark plug wires and plugs, both check out fine on the gtp. I haven't checked the cat. Converter yet, been some terrible weather out here lately, gonna try that tomorrow morning if it's clear out. As for power lost. As they are getting ready to stall, the first sign is typically a sudden loss of engine power, then the noise if I'm off the throttle, and finally shut down. When it's cooled and retired, it runs as if nothing was ever wrong. It's very strange to me.

The good part is:

"Usually I have to wait till cooled off before being able to restart either."

That makes it much easier to diagnose.....

Have a can of carb cleaner? Air temp should be 40F or above when doing this....spray into intake while someone cranks the vehicle....if it it runs as long as you spray, then it's a fuel delivery problem.....either no fuel pressure due to a bad fuel pump, or the injectors are not firing.....check fuel pressure with a gauge on the fuel rail at the schraeder valve....

If it still doesn't start, check for spark.....

On the 3.8, check for fuel in the vac line to the fuel pressure regulator.....also try starting with the MAF disconnected.....also are you losing coolant?

With the misfire code, this causes the system to run rich and can foul plugs....also running the vehicle like this can create problems for your cat, by sending raw fuel into a hot cat.....

How is this for a guess? I am leaning towards a fuel pump for the GA, and possible cat for the GP.....but don't guess, verify first....