periodic misfire

[october12]

My 1973 MGB (de-toxed) has been running fine for several years (I don't drive it very much at all!).  Suddenly it has started to periodically misfire (spit and cough briefly) with an occasional misfire, then runs good for awhile.  It seems to happen more frequently while cruising at 40-45 mph as I let up on the gas or just maintain constant speed.  There are no problems when accelerating. The plugs look good and otherwise everything seems okay.  Altho the gasoline is more than a year old I did refresh it with 7, but the problem persists.  Any ideas?   Thanks!