If it's temperature related...

May seem too obvious to ask, but have you checked your fluid level? That varies with temperature (although, after it's warmed up, it ought to be working), so this is a long shot.

Another long shot ... if the level is OK, how's the fluid itself? Nice and cherry red? Or brownish? Maybe the fluid is no longer clean and some little passage is gummed up? If so, you may have to drop the valve body (not necessarily taking out the whole transmission) for a cleaning?

I'm just mentioning some things that you might find that wouldn't cost you an arm and a leg. Good luck.