I have a 2004 GP GT with 104k. At 40-60 the engine will stumble when accelerating. It gets worse when driving long distances. Last month I replaced the trans filter and put in synthetic fluid. I have replaced the ignition coil packs this week but still have the same problem. I have also recently cleaned the throttle body and had a coolant flush. I replaced plugs and wires about 10k ago. I have not thrown a code yet. Any help on this problem would be appreciated.

In most cases a stumble under load would have me looking at the coils. Generally this is caused by weak spark, so that'* coil, plug wires, and plugs. Check them all. You can also check the fuel pressure.

I checked the fuel pressure today. Key on, engine not running about 45 psi, with car at idle 55-60 psi. The gauge needle did not level out on this but was bouncing while running. The haynes manual I have says the pressure should be 50-62 psi with key on engine off and 3-10 psi less when running. Not sure if this is a problem having less pressure with car off.

I am going to pull the plugs tomorrow to inspect them.

Pull the vacuum line off the fuel pressure regulator and smell inside it. Do you smell gas? If so, the FPR is bad.

The fuel pressure regulator is in the fuel tank. Is there a vacuum line under the hood to do this?

The FPR is on the fuel rail. You will see it with a vacuum line on it.

The fuel rail only has the line coming in for fuel and that is it.

If there is only one line, I'm willing to bet that the fuel pressure regulator is down the line a little ways. Likely at the bottom of the firewall. I know that the 3500s used this setup as well.

No i have done some searching and found that the pressure regulator is in the tank. Heres one link http://home.mchsi.com/~d.traub/fuel.htm.

He'* right, the 2004+ GP'* have a returnless fuel system IIRC and therefore do not have an FPR on the rail.

I'm also leaning toward spark.