2150 Motorcraft carb stalls when I stop

Rough idle, also. I have the idle cranked up to 1500 in park, 900 in drive, trying to over come it, but hasn't helped. It does this in neutral and in drive, but not when slowing, only when the car comes to a full stop. It does not stall when I am just sitting there, w/ my foot on the brakes. It does not stall or sputter/surge on turns or accelerating. It started to do this all of a sudden about 2 weeks ago. There have been periods when it has ran similarly (rough idle/stalling). Idle drops from 650 to about 450 (irratic). It usually lasts several weeks, then goes away and runs great for 2-3 months. During those periods, I've rebuilt the carb, pulled all of the choke stuff off and plugged the holes. I haven't done anything with the trottle shaft, and the float is origional. Car is 1976 Ford Elite, 400m 2bbl auto. I'm trying to avoid large outlay of money because I will be swapping the motor w/ a '71 429 eventually.