Possible vacuum leak that's diluting the air / fuel mixture in one or
two cylinders to the point of misfire. You notice it only at idle
because it's a small leak and at speed becomes a tiny percentage of the
air allowed into the engine. Listen for a hissing sound, check the
various hoses connected to the intake manifold, particularly the small ones.

When is the last time you had your oxygen sensor replaced? What mileage do
you have now? It is quite urgent to fix this as it may damage your cat
converter and that will cost you a bundle.

Thanks for the info, I have 117,000 miles on this one. I also do feel a
difference when running at high speed as well as an idle. Don't know if that
will help explain anymore or not