904 just gave up

My 904 was just rebuilt last May and added a new gear vendor over drive.
took the old girl to town and went around a corner and she wouldn't go for a few seconds so headed for the parts store to get tranny fluid thinking I was low and on the way for tranny fluid it kept slipping or just like putting it in neutral. Added fluid 1 quart then she was fine again so headed back home witch is 20 miles got within 5 miles of my house and she wouldn't go just like putting her in neutral. towed her home and removed the oil pan had plenty of fluid and real clean.
I do have it under warranty but before I take the old girl to the shop is there any thing that might be s simple fix or could it be my gear vendor that's the problem.
And I am not a tranny expert at all.