Engine Stumbles Intermittently

BMW325e 1985, 5 spd, 293k
I need some suggestions about where to look for an intermittent engine problem.
Last summer I replaced my engine (thrown rod cap and holes in sump), with a used engine that came with the wiring harness, and manifolds. Engine runs well most of the time.
Occasionally, the engine will stumble, falter, and sometimes die. Waiting a short while, it regains its feet and is again running fine. Usually happens when slowing, as in approaching an intersection. This behavior is sometimes exhibited at start up, rough misfiring, then eventually smooths out.
The misfiring, stumbling, does not seem cyclical.