Err, for about half of Wrath of Khan
? Or the first third of TMP when he was pissed at everybody?
I don't think Kirk is manic at all in TWOK. He looses his cool for no reason in the Genesis cave, but that's about it. In TMP he's petulant and dumb, but not really manic. But these are both examples of bad writing and I wouldn't use them as references to make a point. The worst thing about those two movies is that Kirk isn't the hero and he's ineffective and wrong most of the time.