...I have a post somewhere on here were mine was doing something similar. Mine was at higher speeds it would kick out of...4th and into 3rd then back up. It was all jacked up. When I took it in I needed a new valve body...I think that is what it was. Apparently there was a defect in the milling or some rott like that. Yellow Coyote I think had the same issue too. They put a transgo shift kit on with stiffer springs and that solved their problem. Do you feel it kick into a different gear?
When mine was acting up I changed my trans fluid and took a look at it. All I had was the usual clutch dust attached to the mag in the corner. All else looked fine. So you could do that and see if there are shavings in the fluid. If so save some of them and the fluid and take it to a shop. My valve body swap and install was at about $1100
If I knew then what I do now, I would have done it myself for about 200-300.