why do vehicles pick the worst times to give problems?

[86Scottsdale]

Well, my X-Mas break is just about over and by this time tomorrow I'll be back down at school (hopefully). Of course, the truck has picked a rather poor time to start acting up, and since it's been snowing more or less nonstop here for the past few days it's not going to be feasible to work on it since I have no garage. Here's the problem, maybe somebody can help me out since I have a few vague ideas but not really the time or place to work it out. As of late it'll start up and idle fine on all of the fast idle cams, but after this is where it gets screwy. Sometimes when it's warmed up enough to come off the cams, it'll drop to about 500rpm in park (I have it set at ~800) and idle roughly, sometimes it'll stick right at 800 and be nice and smooth. And sometimes it'll go to 800 and idle, and suddenly just drop to 500 without me having touched anything. In the worst cases it'll fall to 500 and slowly die out, idle slower and slower and stall. It does it when fully hot, cold, anytime. It's been a little colder here (about 30 deg), so I switched back to the stock air cleaner from the open element, although it used to do fine with the open element even at 15 degrees. That fixed nothing. I put some drygas in both tanks in case I had ended up with water in 'em. That did nothing. Today for instance, I warmed it enough to get to the lowest cam on the fast idle, and drove off. It did fine. I went about 8 miles down the highway at ~55mph, running perfectly, came up to temp. I pulled into a gas station, and as I was circling the pumps it went from it's normal ~500rpm in drive to about 300 and running quite rough, I kicked it into neutral, thinking taking off the load would stabilize it but it still was right down at about 300. I filled the tanks and started it up, started fine, but idled slow, but I managed to drive off. After about 30 min more driving it was back to running smooth again. So, I am rather puzzled. It almost seems like I'm getting a vac leak or something that's intermittant since it'll drop real low like that at a "flick of a switch" for lack of a better term. The only vac lines I have anymore are for the dist, pcv, and brakes, but all are in good shape. There's strong vac if I hold my finger over the end of the pcv valve at idle. Whenever it does stall I can start it right back with no problems. Hopefully I'm making some sense here and not too drawn out. What are your thoughts? My parents are starting to pressure me towards getting a new truck (actually went to a chevy dealer today), but I'm insisting I can get it fixed. It's an Edelbrock 1406 carb, 305 engine, just so you know.

Thanks for any help.