Does this sound like a flow issue? (long)

My DS has been fussing some at the breast lately; he latches and sucks/swallows for a few minutes but then starts to fuss and cry and comes off every few minutes.  It was initially happening on the right side, which is my smaller supply side, and mostly in the evenings, so I thought it was a combination of him being in a growth spurt (he had also begun cluster feeding) and it being in the evening so my supply might be lower, typical fussy period, etc. 

However, he has started doing it on the left side too which has always had a good supply and it isn't only in the evening anymore.  He also seems to need to burp a lot; when he fusses I've been taking him off and burping him and usually get one or two good burps, even if it has only been a few minutes since he began to nurse.  But then as soon as I put him back on it starts again. 

Also, sometimes he sounds like he's gasping while he sucks and when he pulls off he often has milk dribbling out of his mouth, so I know he's getting something.  When he comes off and I burp him, he always seems to act like he's still hungry - cries/fusses, sucks his hands, roots around.  So I always put him back on and then he does the same thing again.  After several episodes of this, I will usually finally stop and get up to walk around with him and he often calms or quiets down.

Oh, and one other thing - in the early mornings when he wakes up to eat (around 3 or 4am) he almost always has green poop - this would be after he cluster fed the night before, during which he almost always exhibits this behavior.

All that said, I never feel engorged and I don't feel let-down; I have no idea whether I've got too much or not enough milk and how quickly I'm letting down.  Reading kellymom's articles it sounds like it could be fast flow or slow flow and I don't know how to determine which one.  Any ideas?

Thanks in advance!