1996 Pontiac Grand Am 2.4L Stumbling on Acceleration

1996 Grand Am 2.4L. New crate engine installed about 15,000 miles ago. Ran great until few months ago. Here's the deal: When I first start the car, it starts fine and then stumbles a bit, especially for the first quarter mile or so. Runs ok, but at expressway speeds acts like it wants to stall then engine surges and runs okay again. Here's the parts I have replaced: O2 sens, fuel pump, fuel regulator and fuel filter. Chilton manual says to check fuel pressure at schrader valve, but on this car there is none. I assume it is fuel related and a pressure reading would help tell me what it is, but can't figure where and how to test the pressure on this car. BTW, reading from O2 sens is.9v steady (too rich) and I have checked all vacuum lines and fuel lines for kinking, etc. Checked TPS and it is reading fine as well. HELP!