Stalling out!

[jgknowla1]

I hope someone knows what's going on with my truck. I've got an '82 C10 305/350.

Couple days ago while driving to work I noticed that the idle rpms were lower than normal at stoplights. On my way home that day, it even stalled at a light. Normal driving speeds are fine - it has fine acceleration, sounds normal, etc. So here's that info:

* When I idle, the RPMs drop slowly over the course of about a minute until it stalls.
* Choking it (manual choke) brings the RPMs back up and it idles fine.
* changed fuel filter - old one was clean inside
* changed plugs and wires
* adjusted idle screws on carb (qjet) to 2.5 turns out each
* looked for and couldn't find any vacuum leaks - tightened down carb to torque specs
* Carb was rebuilt about a year ago and has been working fine

Any ideas of where I should look? What could be doing this?