Does the missing occur at hot idle only or is it while you are driving it, or both?

When is the last time the rotor and dist. cap was changed? If you have the equipment you can do the diagnosis, but if you don't have it, then you have to examine the parts very closely, and many times a defective part is not visibly indentifable. Another alternative is to take it to tech. to do the diagnosis.

You can find a lot the info here, in past posts, just use the search function, in the top right.

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1989 420 SEL