I've been mulling over your situation. But it's hard to diagnose without a code. If you have a code reader and haven't hooked it up, do so and check the pending codes to see if something is tripping the ECU, but not consistent enough trigger the CEL. If you don't have a reader, you can take it to ZombieZone or O'Really's and get it checked for free. <img src="/forums/images/graemlins/notooth.gif" alt="" />

I would suspect that something inconsistent is happening... could be a starting-to-fail sensor (O2 or something in the fuel/emissions circuit), a poor connection somewhere in that system (the wires are notorious for corroding at the base of the connectors), bad spark plugs or plug wires, the timing belt wearing out (stretch or slip, though this is usually a constant problem), or a bad catalytic converter (my primary suspicion - if the cat is worn and the honeycomb/biscuit inside busts loose, it can intermittently plug the exhaust system and cause the engine to run rough and/or backfire). I'm just vomiting some possibilities on the table... <img src="/forums/images/graemlins/barf.gif" alt="" />

I'd check for pending codes, then have a muffler shop check out the cat if there are no codes pending or there's something that points that direction (i.e. O2 sensor related or the fuel system too rich or too lean). You can also whack the cat with a mallet from below to see if something is rattling inside or sounds loose.

Last thought - if you did some work on it recently, go back through to check for loose connectors if you disconnected anything or worked close to a connector that could have been bumped, loosened or the damaged. I've done that a few times... <img src="/forums/images/graemlins/baby.gif" alt="" />

Hope that helps... <img src="/forums/images/graemlins/patriot.gif" alt="" />