I found this guy with the same exact problem...
99 GMC Yukon stuttering
My 99 Gmc Yukon SLT 4x4 4 door with 5.7L v8 Vortec and 4L60E auto tranny with electric 4wd selector, has a problem. It runs fine when you start it up and for the first few miles, then the engine will not go over 1200 RPM without missing and completely dying out. I took it to a mechanic and he/we found a bad O2 sensor that was replaced and it now has an entirely new exhaust from manifold to tailpipe including catalytic convertors. The timing was adjusted (as much as is possible)and all injectors are operating correctly. No trouble codes are set and if you turn it off for 30 secs or so and restart it, it will run fine for a few minutes then start doing it again. It seems to me that it would coincide with the PCM going to "closed loop" but my mechanic is telling me the duration of injectors being opened and closed does not change. I changed the fuel filter and have dead headed the fuel pump to find out the pump generates 90 psi when it has no outlet. When I check pressure at the fuel line test port I only get 50 psi. Is it possible the fuel pressure regulator is bad? The fuel pressure does not change when it is running correctly or poorly. Any ideas or thoughts? I have checked and traced all grounds and have repaired a couple of connections(cleaned and recrimped new wire ends to ensure solid good contact), I have even gone so far as to verify the connections with an ohmmeter. What is the pressure supposed to be on the fuel rail? I have heard and found anywhere from 60 to 75 psi.