2000 gmc 5.7 power loss

I have a 2000 GMC 4x4 5.7 that is intermittently loosing power. Will run great for a while then all of a sudden fall on its face. Struggles to reach 30 mph. I have replaced distributer, wires, cap, rotor, plugs, Map, coil, fuel sending unit. I had an aftermarket auto start system in it. Thought it was causing the problem took it out thought it solved the problem but a month later same problem. When the problem doesn't happen the truck runs great then all of a sudden engine flutters and dramaticly looses power. Thought the cats may be plugged took it to the muffler shop and they told me if they were clogged it would do it all the time not intermittently. Any suggestions ? I'm stumped