if it was a vaccuum leak, the idle would go up, down, up, down, up, down, etc. If the idle just drops low when u stop, check the distributor and the MAF. I had the same problem for a while. It would drop to about 500 and shake like it was gonna stall. weird. check all the obvious, spark plugs, wires, cap, rotor. and def. check the distributor for oil. that very well could be the problem.