5.3L running very rough...

Okay, I've got a problem with my '99 Silverado with the 5.3L V8 and this is gonna be kinda hard to explain but I'll do my best. A while back the truck started idling pretty bad at times, and its wants to lurch forward in a very jumpy manor when I barely take my foot off the brake. The Service Engine Light came on and I took it in to the mechanic(hes worked on my families cars for about 10 years so hes not just some sleezeball). Well he replaced the plugs and wires and ran diagnostic test on it to no avail.
The only thing that he got from the diagnostics was the 4,6,& 8 cylinders(may have been different cylinders but i know it was 3 of them) were reading inconsistently each time. He checked a few more things out and couldn't figure it out. I'm pretty sure he checked the injectors as well. Well last week it started running even rougher on 2 seperate occasions and the same thing happened yesterday. When it runs how it has on these 3 seperate times the exhaust note and overall engine sound, sounds really wierd(i wish i could describe it). Yesterday though the Service Engine light began blinking on and off around 10 times and then stayed on. But once it drives for a few minutes(around 5-10) the problem seems to go away.
The roughness only seems to happen when I put the truck in gear(i.e. Reverse and Drive). But in park it goes back to idling normal. This is really starting to scare me b/c I have no idea what its doing. I would love it if someone knew just a few things that it could be so I can maybe suggest to the mechanic to look at these. The truck has around 58,500 miles on it. I hope someone can help. Thanks in advance if anyone can!

You have a PM..

I'm having the same problem, but with a 6.0 V8 2500 Silverado. When the engine gets warm and I come to a stop while in Drive, the idle goes revs from 400 to 700 and makes a rattling/fluttering sound. Put truck in Neutral - it stops. It also stops on its own after 30 seconds.

My mechanic can't figure it out. Can anyone help?

First thign I would check is the vacuum line leading to the fuel pressure regulator. Pull it off and see if it drips fuel.

vacuum line looked OK. Intake manifold?

I had a problem with my 1999 5.3 liter running rough on and off when I first started it up. Here is the problem. There is a gas vent solenoid switch located between the cab and box on the driver's side. According to the GM service bulletin, the switch is defective. It allows water to enter and collects it in an evaporator canister located at the same place. This one looks like a rectangular black plastic container 7''x10"x3". It has 3 hoses going to it. One goes to the switch, one goes to the gas tank(vent), and one goes to the motor. This canister gets water logged and as gas is used in the gas tank a vacuum is created so it draws from the canister, thus sucking water into the gas. It then flows down the gas lines and into the fuel rails. When the engine is shut off, the water settles to the lowest point on the fuel rails and sits right on top of the last fuel injector nearest the firewall. When you start it up you are injecting water into the cylinder instead of gas.
I would suggest that you put at least a couple of cans of dry gas (gas line antifreeze) into your tank and see if your problem improves. If it does, I would replace the switch and canister.

I also have the same symptoms in my 99 silverado. The RPMs will fluctuate between 500 and up to 1000 rpms.. We took it to the dealer who we trust and none of us could figure it out for the longest time. He suggested we change that solenoid, so we did. Didn't help. Finally, the dealer had the same problem with a tahoe with the exact same engine.. So he came at my truck with a can of starter fluid. he sprayed it at the intake gaskets and it almost died. It turns out the intake gaskets are bad. Since it happened to that tahoe, I'm thinkin there's a defect or something in the manifold. I've replaced the gaskets, but it still sputters when sprayed with ether. So I was wondering if anybody knew of any recalls or anything to do with that..
Any help would be good
Thanks