99 4x2 5.7L stumbles

Recently, my 99 SUB fuel pump died. Replaced with aftermarket unit. Truck started fine and ran great for about 200 miles. Now, it runs for about 30 minutes and then stumbles. Tried fuel injector cleaner, then a new filter and a new coil. It also has new wires, plugs, rotor and cap within the last 6000 miles. When it starts to stumble, it will continue to run, but a sever lack of power with RPM's increasing with more throttle along with a slightly faster speed. The exhaust sounds muffled. The SES light never comes on and there are no codes present. If I pull to the side of the road for 15 or 20 minutes, it runs fine again for 30 minutes. It idles fine, gas mileage isn't suffering. Any help before I have to succomb to the dealer to find what is wrong. Thanks.

This is a wild guess, but perhaps the vent is plugged. I'm not sure of all the routing on the vent lines -- probably goes through a charcoal canister somewhere -- but it may be possible as the fuel level drops in the tank and no air goes in to equalize pressure, you could be pulling a vacuum on the tank and preventing fuel from pumping. Over time, the pressure can equalize, but not as fast as you're using fuel. Again, this is a wild guess, but you could check it out the next time the vehicle stumbles, open the gas cap and see if air goes into the tank or out of the tank. If in, then you've figured it out. Good luck!

Thanks. Haven't had the opportunity to try this yet, but I will this weekend and will post my findings.

I found the vent hose on top of the fill hose and corrected that and drove the truck. It was okay for about 10 miles then started to do the same thing. I took the fuel cap off and didn't hear any air go in or come out. I drove another block with the truck stumbling and pulled over again. This time, I just left the cap off and had no better result. At the next traffic light, I found that if I pulled the shifter into 3, it would run a lot better and then put it back in D, that worked for a while. After about 40 miles of stop and go, the transmission started slipping in 3 and hardly moving at all in D. I limped it back home and waiting to take it to the transmission shop that rebuilt it about 4000 miles ago. I don't know how the two are related, but I hope this will fix it.

your catalytic converter may be fouled out. My brother-in- law had a very similar problem with his trans-am(5.7). He would drive for a couple of miles and it would start to stumble and default to limp mode. He changed his converter and everything worked well. Hope this helps.

Thanks for the replies. I picked the truck up from the transmission shop the day before yesterday. They tried to tell me that one of the plug wires was arcing in the exhaust manifold and because of that, the transmission broke. They had to replace the third gear clutches and a band along with the torque convertor. I drove home, approximately 10 miles with no issues, and I drove it again yesterday about 30 miles with no issues. In no way do I believe a plug shorting out will kill the transmission, but that's the story. I plan to put about 50 miles on it this weekend to see how it does and I will post those results afterwards. Thanks for all of the inputs.

Well, I have over 100 miles on the suburban since I picked it up from the transmission shop and it has run flawless to this point. I plan to drop a line to Mr. Transmission's corporate office today to let them know of my displeasure with them not fixing my truck under warranty. The good news is the truck is back on the road and I feel confident that I can drive it again. Thanks for the help and keep it between the lines.