Hesitating/backfiring

["wcameyer" <wcameyer@email.msn.com>]

Hello, List:
I need help.  Twice recently I've driven 10-15 miles, my '77 B rdstr running
beautifully at speeds up to 85.  Then it is parked for 1/2 to a full hour.
On the way back, after a while, it will hesitate, buck like H---, and
possibly backfire between 60 & 70.  On Father's Day, it ran like a top for
200 miles at speeds up to 90.  Standard engine, Piper 270 cam, Weber
downdraft.
What's going on here?  What should I check?

Bill