Tune-ups usually fix a constant problem, but if it's been a while, it definately won't hurt.

Have you checked to see what your plugs look like?

Your problem sounds very temperature related. Could be a poor connection, a bad ground, poor wiring, or bad engine temp sensor (among other things).

Next time it's running smoothly, wiggle wiring & tap on stuff - especially modules and sensors. See if you can get it to stumble.

You could also start it up, and if it's running smooth, turn it off and take a hair dryer to a control module or two, then start it back up. If it starts rough, try a little upside-down air duster on the same modules then start it back up to see if it smooths out again. This is basically the no-tools heat-related problem approach.

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A.S.E Tech A1,A6,A7,A8 & MVAC 609 + EPA 608

Unless stated otherwise, any question I ask is about my greymarket 1985 380SEL.