99sub rough idle

I bought a99 suburban 2500 7.4L last August in texas. Ever since I got it,the idle has been rough. It started out as just kind of a lilltle hiccupevery five seconds or so. While it was under warranty the dealer said that all big blocks do that . They had no interest in looking any farther. Ilived with it for a while but it seems to be getting worse.i.e if I sit at a stop light for more than two minutesthe idle will get real rough,and then the engine dies. Itook it to an independent shop,and they suggested atune up.I did the plugs (bosch) GM wires rotor,and cap. Nohelp. While I was doing the tune up I found a small intake manifold leak,so I changed the upper and lower intake gaskets. Nohelp. hile I had the intake off I ohmed out all the injectors. 12ohms each. Ialso replaced the coil and ignition module. No help.

Itook the truck back to the shop to have them trouble shhot the problem. They put the csan tool on, and found a random misfire on all 8 cylinders. They thuoght there might be a vacumn leak, but they could only find one little one . They fixed tha tbut still had the misfire. I'm out of ideas, the shop can't figure it out,and I'm wary of going to a chevy dealer ,bcause I've had a number of bad experiences with dealership service advisors lying to me.

Iread "brennicurn's" thread with great interest. I called chevy,and they said that, that was only for vehicles with C.A emissions,and I should try another chevy dealership. Any help would be greatly appreciated.

First thing would be an injector balance test..I have had 2 7.4 engines with rough idle and misfire complaints caused by clogged injectors....fine sand particles got past the filter and stopped up the screens on the injectors.
Flushing the fuel tank and lines fixed one..the other was a wrecker that the body builder drilled a hole through the filler pipe allowing dirt to wash into the tank when it rained...wierd but true.

Thankyou for your reply,
I forgot to mention that I had the injectors cleaned, and put a can of "black magic" dealership fuel injector cleaner in the tank. I dont put a lot of faith in chemical cleaners so that is deffinitely aplce to start.

thans again for your help. It is greatly appreciated

Unfortunatly snake oils wont work in this case..the screen on the injector is on top of the injector to prevent the dirt from entering the injector.
Removing the injectors and cleaning the screen or replacing the injector is the only true fix.

thanks again Iwill give it a try

Make sure you do the injector balance test first!
this will confirm if the injectors are flowing the proper amount of fuel.

It was the plugs! I did finally find a chevy dealer with agood service dept. They did the fuel balance test, every thing checked good . Put in a set of ac delco plugs and now she runs 100% better. Bosch just dont work well with this motor I guess. Thanks again.