my truck did that 3 times with no real reason as far as I ever found. I put on a carter, the fuel stopped leaking, but it ran like crap, I put on one of Tom Landgons little 2 barrel, it ran poorly, I wedged the heat valve in the opposite position it was in, and that really made it run bad. I changed plugs, wires, rotor, points, condensor, gas tank, fuel pump, fuel lines, timed, retimed. Nothing helped, so I put the original rochester back on, it still ran poorly, then one day it just started running fine, like it had for 25 years. Never did figure it out.
