Maybe a broken vacuum hose to the distributor? Would be the easiest guess, sometimes it's the little things... I had exactly your problem and traced it to a cracked hose. you wouldn't even see the crack, but the moment you rev suddenly higher, vacuum gets higher fast (if your on ported vac), that's enough to drag a bit of air through the crack, f**k up your timing and get the engine into low revs again. when revving low the whole game starts over...
JMO, Max