My little brothers 95 z28 4l60 is having a problem. Under normal driving it shifts perfect and even under 80% Throttle it will still shift fine, but when you hit 5000 rpms the trans just freewheels. It is almost like it gets kicked into neutral. The trans does not slip any other time when driving just over 5000 rpms.

Is there some sort of solenoid or relay that could be causing the problem? The fluid was just flushed and the trans filter replaced. The car is bone stock. I really don't think this is a internal trans issue because it drives and shifts ok even under "spirited" acceleration, just not over 5k rpms. Does anybody have any ideas? Thanks.