As you say, you're fundamentally shifty and from my observations, some of them seem to subconsciously enjoy it and I've known them to sort of..."play that up" and make it work for them. Again, not to be misleading or manipulative, it's just how they are. Similar to how the NTPs will "play up" the haphazard, socially broken eccentric. However when the need arises to make a purposeful, non-ambiguous impression you're at a loss as to how to do it properly and as you say, overanalyze all possible interpretations of events, try to protect the Fi core, fumble about, fear the Se, repeat.
INTJs can't help but look at things strategically and part of "winning" the game is to cloak your moves, your intentions, what you know; generally just being obscure. It can be difficult to turn that off when you want to, and regulate it so as not to give mixed signals.