Help! 99 Tiptronic slipping when warmed up

[engarom]

Ok so i have a weird issue.
I can drive my 99 boxster fine in the morning, as time goes by, maybe 15-20 mins, i start to notice the gear changes getting "loose". the engines revs higher between the shifts.
as i drive more, i can feel the transmission slipping more and more to the point when i go home i could not even climb a very slight incline and it kicked into limp mode.
i parked the car and waited a few hours for it to cool.
wouldnt you know it, i came back out, started it up and climbed the hill no problem.

So, I thought maybe low or bad fluid. I did a full fluid and filter change, drove it around and same problem. Transmission slips more and more the warmer the car gets.

No check engine light, no other issues. As soon as it cools down, it works again.

Anyone seen this or have any idea what is going on?