About 50% of the time when the truck is cold, the engine will stumble for a second or two at startup. It then jumps right up to fast idle at about 1500 rpm. It never stalls and only does it about 1/2 of the time. Any ideas of what's causing this? I did a full tune up a month or so ago and it still does it. It runs great otherwise.
EDIT: When it's warm it starts up fine.

Have you cleaned the throttle body?
Those things can get very dirty over time.

I did recently clean the TB. It wasen't very dirty though.

You might try the fuel pump test jumper. Could be the fuel pump is sometimes not run when the starter is engaged and the stumble is due to the drop in residual fuel pressure before the pump can catch up.

Keep an eye on your coolant level...if there is the slightest drop then gear up for a cylinder head removal. I've learned that the symptoms you describe have been a blown headgasket (2 occasions) and a bad head on another.

That stumble is one of the cylinders not firing, the stumble goes away and the engine resumes normal operation when the cylinder dries out.

Hows this happen when cold and not warm? Slow coolant leak, as the engine cools down the seepage is occuring...residual leakage is at first evaporated till the cylinder cools down enough...leak still occuring. As casting cools, crack seals and stops seeping coolant. You now have a wet cylinder. Wont happen on a warm engine because there isnt enough time for coolant to collect before refiring.

~Darin <img src="/forums/images/graemlins/baby.gif" alt="" />

Doesn't seem to be using any coolant. The level has not changed in the 2 months I've owned it. And it was doing the stumble when I bought it. I'll pull the plugs in the morning and see if any of them are wet with coolant.

It sounds stupid but have you double checked the cap and rotor?
Mine had a mystery stumble happening. I tried alot of stuff valve adjustment checked compression it was all good.
It turns out when I put the new cap it wasn't aligned all the way and the rotor gouged all the points on the cap it also gouged part of the rotor off.
Pretty common issue the GP Soreson caps don't fit exactly like OEM so they have that issue.
I changed out te cap and rotor and made sure it was all straight when I put the new one on and now the stumble is gone.

I pulled the plugs this morning when the engine was cold. No signs of any coolant on them. All looked nice and clean. I changed the cap and rotor when I did a full tune up last month. The stumble was there before and after. It only does it half of the time and only for a second on start up when its cold. Any other ideas? Its not a big deal but I'd like to fix it.

Have you tried the fuel pump test jumper?

test fuel rail pressure