X5 3.0D 2001 transmission slipping

[Thelittlefatguy]

Hi everybody, New to the forum, and im having a bit of an issue with my x5. Yesterday after a 600 km drive(No trailer attached), 2 days ago, the car went i to transfail safe mode about 100 km from my home and staying in 5 gear. Pulled over to quickly inspect under under the car, but found nothing, and continued my journey home without shift issues.
About 50 km. later, same thing again, and I redo the same. Fire it up again without problem. This repeats a couple of times the last 50 km.
Near my house is a hill. The car started to rise in rev(up to about 2500 rpm) but didn't move faster, and slowly started to losing abit of speed, but made it over without any drama.
Yesterday i went for a small drive around town, to see of the issues was the same. Initally there wasn't an issues. Shifted nice without problem. But after 5 minutes, failsafe mode came back, the car couldn't move up a small hill. Put it into neutral and backed down to side of the road. Waited about an hour to let the trans cool down. Fired it up and drove about it about 500 meter before the problem was the same and the car wouldn't move. My buddy came and towed it a couple of km. home.
Any thoughts?
Oil and filter was changed about 13.000 km. Ago and the box has maybe gone about 80.000 km, since it was rebuilt. Gearbox is dry underneath so doesn't leak aparrently.
Im open for ideas.
Per.