124 runs rough after freeway runs

I had a problem that started today, after a freeway run I pull off the freeway up to the light and it's running as rough as can be, you'd swear it's a big vaccum leak. after about a mile or so of city driving it clears up and it's just as smooth as can be. Gas milage is good, temp. is fine. I did notice the smell of a hot cat. converter on a uphill run, I'm not positive it was me. By the time I get home everything is fine.
Any ideas, suggestions? Tomorrow I will get it up on the freeway and pull it over and look under the hood, see whats up.
Thanks for any input,
Chris C.
86' 300E
152,000 miles