'96 Lumina - misfire?

A few months ago my wife's '96 Lumina started running rough and the service engine light started blinking from time to time. It's got 135k miles, so ran a few tanks with injector cleaner. The problem seemed to clear up but now has returned, including the blinking service engine light. I seems like it may be the timing, but I'm not sure how that could have gotten off. Any ideas or suggestions?

Thanks.

The best thing to do is to go get the codes pulled from Autozone or somewhere that will do it for free, which will give you a good idea what your problem is. From you description and the miles it has, my first guess is that you have a bad injector.

No provisions for timing on these. Dwal's probably right, you might have an injector dripping or something like that. If the code stored relates to over rich condition, the injector might be failing to close. If lean misfire, there may not be a full spray pattern coming from it. Does it seem to come from one cylinder (rythmic miss), or just random misfire? At idle, or through the whole range?

Thanks, guys. I'll have the codes checked. The "missing" is fairly rythmic, most noticable at idle and seems to improve with acceleration and speed.