Before you get too involved looking for elusive stuff, did you consider worn spark plugs?
Four pounds low on fuel pressure will not be an issue on other cars, but to rule that out on a GM product, when the rough running is occurring, look for the fuel return line where it is rubber between the fuel rail and the body. Pinch that off with a hose pinch-off pliers. Fuel pressure will increase if the pump is able to develop that higher pressure. That will prove the pump is okay. Those cause a lot more trouble than regulators do.
GM has had a lot of trouble with their fuel pressure regulators leaking, but that also applies more to truck engines. Pull the vacuum hose off at the regulator and look inside it. If it is wet with gas, replace the regulator. Pulling the hose off should also make the fuel pressure go up. If it does, the regulator is working.
Now for the more elusive cause of rough running on GM vehicles. Unplug the small one, two, or three-wire plug from the back/side of the generator. If the rough running clears up, I will discuss that in more detail.
Thursday, May 4th, 2017 AT 8:09 PM