Sounds to me like maybe the vacuum advance isn't working correctly, could be a vacuum issue or a distributor issue. Depends on the miles on it. Might just need the timing bumped ahead a little.
There are a lot of variables that could be causing the problem. Carb. going bad. vacuum leaks, vac. advance on the distributor going bad.
Have you put a vacuum gauge on the motor? 18-20in. HHG is optimum for a 4.2L
That's how I time mine, I use a vac. gauge to get optimum vacuum, then adjust my idle to get a 750-850rpm.
'87 YJ, 4.2L built and bored, 8" of lift, 8.8 rear, AX15/NP 231J, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...