1993 Subaru Legacy Please Help

Yes, I am having a little problem with my car. It is a 93 Subura Legancy. The car will idle fine until I put it in gear and when I mash the throttle it will hestitate and stumble. The mechanic checked the fuel pressure it is about 25 and is supposed to be 36. We have put 2 fuel pumps on it so we know that's not the problem. Could it possibly be the fuel regulator. It has new plugs and wires also. The mechanic thinks maybe the timing and of course that is more to get into than your thougt. I drive the car for delivering the mail. If you could possibly give me a thought. Thanks!