I think all the causes you list operate. (And I think the occurrence and sources of exhaustion would be worth exploring extensively. There are probably interesting interactions between that and all the other factors you list.) Note that you concentrate on the "equilibrium," understandably so because there exists real difficulty in identifying any principles that characterize the (infrequent) punctuations. It seems to me that such periods are seriously unpredictable and probably can't be forced. I've got to do a lot more rereading of Gould to get a grip on this, but I think he claims that in evolution too the punctuations are mostly contingent and quite unpredictable.
If this is so, then it should be possible for "the left" to avoid a good deal of merely destructive self-laceration (called criticism), since such criticism seems implicitly claim that "things would be better" if "the left" would shape up. There _might_ and might not have been something that leftists could have done in the period of (say) 1970-74 to carry the movement of the '60s forward, or even reignite it. But I myself am convinced that nothing leftists (or _the_ left) might have done in the last quarter of the century would have made any significant difference. This doesn't mean that we should have packed up and gone home. One keeps digging, in part because the next explosion is unpredictable, but the main task during such periods should simply be not to burn out and to maintain some sort of visibility.
I guess another way of putting it is that the question, "What Is To Be Done?" is a question that is only intermittently relevant -- the answer to it most of the time being simply, don't tire out and don't get frantic. (A friend in the '60s quoted Lenin as saying there are three revolutionary virtues, Patience, Patience and Patience. I've never come across it in my reading of Lenin, but if he didn't say it he should have.)
We entered some sort of period of growth, I think, on 9/11, and I've argued locally that our main obligation now is to hold together as best we can for the time being: that nothing very explosive is going to happen, but we should maintain exploratory activity so as to remain visible.
In 1968 there appeared to be a chance of a real merging of the anti-war and civil-rights movements. (Did the King assassination stop that?) Perhaps a sign that we are in a real period of punctuation is if different struggles do began to overlap or merge significantly. That isn't happening yet in the present.
And perhaps a sign of equilibrium is if too many activists desert political activity to line up behind the Democratic Party, which always I think is a signal of giving up.
Carrol