1987 L6 with engine running poorly

My car is 1987 635 (L6). It would not start, only a click. Mechanic changed starter and new battery, all was well. One month later, same problem, only a solitaire click when you turned the key. Replaced the starter again, still only a click. Mechanic said the engine appeared to be frozen, they pulled on the belts to free the engine, and the car started. But now the car idles and runs very poorly, like out of timing. mechanic thinks maybe the timing chain had slipped, was frozen, and then jumped?
Sound logical? Any suggestions for other causes for the irregular engine operation?