I'd consider the possibility that the timing has finally gone out farther than the engine computer can compensate for. Mechanical wear. If it was originally driven rough, then it probably wasn't serviced well, and that cuts down the lifespan significantly. If you know a good import mechanic (and there ought to be about six million in your area), they'll usually give you a free inspection/estimate/two cents worth.
One things for sure...it won't get better on its own.
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TinKicker
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