Got a good one for ya here:

So I changed my fuel pump gasket, valve cover gasket, a rocker arm, and two push rods the other day. Get everything back together (including re-install of the EGR valve, plug wires, and vacuum lines since I had to take them off to get the pass valve cover off. Marked both the plug wires and the vacuum lines prior to disassemble). So I get everything back together and now the truck won't hold an idle. It will run with your foot on the gas but will not idle and just dies when you take your foot off the gas while in park. I look around and notice the PCV valve has come out of the grommet and is sitting on the valve cover. Go to re-install the PCV to see if that was the problem and it breaks. Go to buy a PCV valve and am informed that the two hose PCV is Cali emissions and the local parts store won't be able to get it for 4 days.

Since I'm not convinced that a busted PCV valve would create the malfunctioning idle situation I continue to troubleshoot. The two guys that were helping me convinced me that it smelled like it was running real rich and that we should try and adjust the fast idle on the carb. Had a bad feeling about messing with the fast idle adjustment but I figured I had nothing to lose. Made the adjustments and nothing, still wouldn't hold an idle (keep in mind the PCV valve was still busted so whatever vacuum leak that was creating was still present). We were able to fabricate the a "fixed" version of the PCV valve with epoxy and re-installed. Still wouldn't hold an idle. Kept trying to adjust the fast idle and nothing. Only runs (and runs rough at that) is when you are giving it gas. As soon as you let off the gas the engine chokes on itself and stalls/quits running.

Any thoughts on why it won't hold an idle now? BTW, called a different store and the PCV valve will be here in a few hours.

 

the pcv isually going to a large port on the intake which will cause a huge vacuum leak. you can just plug the vacuum side and leave the pcv out for the time being as it wont hurt anything

 

Does this continue after it has warmed up, I know it can take awhile to warm up, when you have to manually apply gas, but if it eventually stays running(albeit rough) then it may suggest a choke issue, I'm guessing you have an electronic choke?
That is a possibility, but it does not rule out many others, this can also happen if your not firing smoothly, or the timing is off.
Other thing to check; spark plugs, are they in snug, cables- are they in good shape( and even though you marked them, are they in the right place(this can happen)) know your firing order and make sure they go around distributor cap accordingly, distributor cap, and check your timing.
I had the same problem, and after servicing the above mentioned, it is doing a lot better(although I have tranny issues, which are effecting it from running perfect). So I would like to believe that if this helps you, then I did in fact resolve MY issue, and am correct in thinking it's current state is just the tranny issues I think they are.
Just some suggestions, I hope it helps some. Give an update when you finally figure it out, I'd be interested in knowing.