I think everyone is too focused on the valve. Generally, when I think floating valve, its a problem that tends to happen only at higher RPMs. Which is not the case here. If a spring is broken, I could see it happening all the time, but the OP states that his truck can run fine for months at a time, so "valve float" isn't very convincing. There may be a bad componet..spring, lifter, rocker...so how about doing a compression test to verify whether the valve is sealing or not?

OP...when you replaced the plugs, did you notice any inconsitencies with the plug colors? Did the look sooty or burnt? Have you ruled out a bad fuel injector? Does your oil smell at all like gasoline? I think there are many other areas that could also be potential problems.

While the 30 year mechanic could be right...and may in fact be right, but it sounds like his diagnosis was soley based on your description and I know I've heard some very misleading descriptions that lead to poor diagnosis before. So, all I'm saying is lets not focus soley on the valve.

Disclaimer: I'm not a mechanic so take my advice as you see fit.

BTW, 2001 2500 5.9l with 180k and running great.

 

thanks fivspeed for the insight and imput. when this issue started early last year was after i did an intial tune up on the truck (ie. plugs, wires,cap, and rotor). Well after rerouting the #5 and # 7 wires per the forum it kinda went away but the stalling during warmup stayed ( it was there when I bought the truck). We did a compression test on #5 and it was good, we switched the # 5 inj with the #1 to see if the misfire would follow and it did not it stayed at # 5. I went ahead and changed the wires,cap, rotor, and plugs again and the plugs showed a lean like color ( light biege color). We checked the fuel pressure of the new fuel pump and it was normal at 49 psi. I am taking a day off this friday to take the truck to my best friend's shop and have them go over it again. We are going to recheck everything again ( ie. injector, compression, valve wobble, spring, etc.). Now 1 thing I'd like to note is I took the advice of 1 of the respondants to this problem and kept double dosing my fillups with fiel injector cleaner and the CEL went off by itself however returned when the tank got down to about 1/8. I drove with it on however at the next time I started the CEL went off by itself and since has not come on. This is the biggest mystery I have ever had with this truck. I am going to let everyone know what it found , or not found, after friday. The truck runs great and this miss and CEL only happens at start up especially start up when the truck has set for a few hrs and during idling. Once the truck warms up and its being driven it smoothes out. At a stop lite I can feel it a little. The the truck will surprise on the next start up the CEL will be off.