Shark,
I experienced the same thing earlier this week.
See this thread:
http://www.peachparts.com/shopforum/showthread.php?threadid=61827
The day after my oil change I made a run from Austin to Corpus Christi. For approximately 95 of the 120+ miles of IH37 south of San Antonio I was running around 85 mph and on the return that afternoon about 80 or so.
I been wondering if the prolonged high speed driving coupled with a full crankcase didn't sling a little extra lube oil someplace weird in the breather where it could run into the intake manifold after shutdown.
Whatever it was, she isn't smoking at start up anymore but the rough idle is still there until the engine has run for 90-120 seconds.
Strange old cars, aren't they?
RL