Vacuume issue! Power brakes? If so check the line. If not you may have an internal vacuume leak on the intake gasket. A V8 will not run on 4 cylinders on the highway. Pull all 4 plugwires and try to drive it.
66 Dart GT, 402 11.18:109 Best
63 1/2 Galaxie 500XL 406 4Speed 13.20:103 Best
2000 Ram
2005 Durango Hemi.