In the morning, sometimes when I start it up for the first time, it will turn over fine and idle smooth for about half a split second, then the RPMs will drop to about 300-200 rpm and run rough as hell, almost stall, then after 1-2 seconds catch again and rev up to around 1200-1300 for a few seconds, then drop down to a normal 800.

Bizzare, eh? It's stumped several mechanics. It's happened for the past 3 or 4 months, and I've noticed it'll do it a lot in cold weather, so I'm thinking maybe the 02 sensor(s) aren't heating like they should, so when the truck starts they start sending false readings which chokes up the engine, untill the exhaust warms them up, when they start working right. Other than that, I don't know what it COULD be, the entire ignition system is ok, air intake is ok, filters are all new, fuel system tested ok, and the starter is fine.

 

The O2s aren't used immediately after startup so you should be able to rule them out. I'd say that the most likely thing to cause that would be a sticking Idle Air Control valve.

 

a vacuum leak is a possibility, check around for those

 

first thing i would think of is an old fuel filter, since it's a common cause for such symptoms. but since it checked out, i agree with bob about the IAC valve. dan (black-n-blue) had the same thing happen recently and it turned out to be the IAC.

 

long shot but my 94 3.0 did the exact same thing and it was the transmission vacuum modelator was bad and leaking tranny fluid right into the a group of about 5 vacuum lines that were oil soaked and it was sucking the tranny fluid right into the motor and causing it run horrible at idle , got new tvm and vacuum lines and runs awesome now

tmv was only 12.99 also