tranny released to neutral on overdrive on 80k/hr

[chevmusmaz]

Hello i owned a 1991 mustang gt. When car is on overdrive gear moves up to 80k/hr then goes to a neutral but then if goes down to 60k/hr goes back again to gear. Their is no weird noise or weird noise when the gear shift .drive gear is perfect i can drive it on highway with no problem. Can someone tell me if this problem is an overdrive switch related or if so. Does the overdrive switch located outside or inside the tranny. I appreciate your help and thank you so much..