Hi, Im a newbee on here. Im Mike.

As usual, ive joined cus ive got a problem.!!Hope someone can help or has some ideas.
My Z71 vortec 5.3 is a 99 model. Has always run fine and started great. We disconnected the rear o2 sensor, in the exhaust, behind the cat,so we couldfit a new stainless exhaust. We removed the old exhaust. Then, had to move the truck, and started the engine to move it. It ran fine and we moved it 30 yards. The next day we started and moved it back in the work shop. The engine then stopped. Umm, says I. I wonder why that happened. Spent loads of time and about a week trying to start it. The new exhaust was fitted and the o2 sensor re fitted. Im sure its not broken or faulty, tho it could be.
Its been back firing out the exhaust and intake. Seems like an ignition / timing problem. ! Yesterday it started. Wheeeee. !! Ran for a few minutes and seemed to drop down on to 6 cylinders and revved a bit weak. Turned it off. Struggled to start it again. Ran on 6 ish, again. Didnt drive it. Just let engine run a bit. Didnt seem right, tho the exhaust sounded nice. !!!
Today nothing again. OBD reader and the Superchip 2815 both say no fault codes. There are new plugs in it and they are wet with fuel. Pump is charging with ignition turned on.

Im struggling.; Any ideas. ???

With thanks. Mike.