Have you checked crankshaft endplay? Really worn thrust bearings can cause strange clutch behavior. I can't remember whether Volvos of this vintage used diaphragm type pressure plates, but, if they do, then you should also inspect where the throwout bearing presses on the diaphragm fingers. The fingers can get thin. I suppose incipient failure of the clutch fingers could cause symptoms like you describe. Incidentally, I think George's idea is the best one. I have seen pieces of clutch facing, or clutch hub, (I forget which) interfere with declutching.