I have two bicycles that suffer this problem, both pretty heavy frames (one a ladies, Hollander bike, and the other a man's commuter) - both have correctly adjusted Derailleurs which don't produce any noise or rub on the chain during normal riding.
However with both, on flat ground, at nominal speed - once every 3-10 revolutions of the chain it will simply skip; there's no drive for a quarter of a turn and then everything is OK.
I had originally assumed that this was a misalignment, or normal chain-skip (as I had only noticed it under heady load, cycling hard out of the saddle. However noticing the same happens on my girlfriend's bike, and on mine when I'm taking it easy disconcerts me, and leaves me with little confidence in the bike.
I'm not sure where to start diagnosing, however I suspect that stripping the drive train might be a start… perhaps also examining the chain for nipped-links, and the sprockets for damaged teeth (although, both bikes have a bash-guard on the front sprocket)
Any help or advice greatly appreciated, I'm a pretty handy motorcycle mechanic, but I always found working on my Human powered bikes a lot more fiddly :)