A couple of things it could be, first(easiest) check your vacuum system for a leak, secondly if all test well check your headlight switch under the dash there are 2 vacuum lines there the switch could be your problem, and finally(the hardest) if not those 2 there is a vacuum diaphram in the dash behind your radio and gauges it is behind the vent tubing toward the drivers side it could be bad. It definitely sounds like a vacuum problem I have a 1981 and constant fought vacuum problems. A suggestion if you have the $ and time is to go to mamotorworks.com and order their vacuum system replacement kit it comes with diagram. not hard it is color coded but it is time consuming but it fixed problems I didnt realize I had