Working on a friends Z 81zxt. The timing is all over the place. Set it to 20 and it runs great,then 5 min later very bad(backfires and sputters)and the timing is off +/- 10deg or so. At that point you have to just turn the dist untill the car runs good, sometimes set to 30,40 or even 10deg. . We changed ecu and dist several times no change. You can actually watch the timing change; any ideas?