My 97 Burb w/258K miles is stumbling/bucking mostly when under load or slight acceleration. No codes. Smooth idle. Seemed like fuel issue so I replaced fuel filter and no change evn though i had the same symtoms 2 yrs ago and the new filter was the trick. Not this time. Checked fuel pressure and it was 55 psi at idle and reving the engine to over 3,000 rpm it never dropped below 52 psi. Shut engine down and pressure dropped about 10 psi in one minute. Do you think the fuel pressure regulator could be causing this since this is over the 5 psi in one minute I read somewhere on this board? What other likely causes? O2 sensor(s), plugs/cap even though not enough misfires to throw code? Other ideas?