Truck stalls at idle

I have a 1977 dodge D100 1/2 ton pick up. The engin was rebuilt about 4 years ago and I repplaced the carbarator about 3 years ago, and that carbarator (at a mechanics recommendation) was rebuilt about a year ago. Anyhow, the truck began to act a bit tempermental at low rpm about a month ago, but it was an off and on problem, and I thought that perhaps I had some water in the gas as it seemed to clear up after a few tanks of gas,for about a week it was running fine, then abruptly it went from running great to stalling at idle speed, it stalls in seconds. If I take my foot off the gas it is just like turning it off. Any ideas?

Fuel pump?

well it has a mechanical fuel pump, would it go out like that , where it would still work at higher speeds?
I realized I left off some info, when it was rebuit it was overbored a bit. It has a carter 2 barrel carbarator.

No ideas?

I have some new info… the truck seems to run fine for the first mile or 2 and than it starts this problem. As soon as it starts to warm up it stops working properly.

still nothing? I am running out of ideas. Anything anyone?