[D13]

83 would be an integral head if a 250. Notorious for vacuum leaks and a finicky carb. Check / replace every vacuum hose ( probably due anyways) but more likely the carb needs to be addressed. The 2bbl's on these are touchy on being clean and also a little tricky to adjust. My guess is dirt in the carb. It doesn't take much to plug those itty bitty holes.

The other thing that will give you this is a distributor with worn out bushings.

One of the quirks of the six is they will run under fuel and timing conditions that would cause every other motor to die, which can make it hard to diagnose. But my bet is in the carb.