!994 Chevy Truck?

Driving down highway speedometer will jump back to zero and the truck seems to lose power for a fraction of a second then the speedometer will jump back up to the correct speed. I connected a radar detector into the cigarette lighter and when the speedometer jumped back to zero the radar detector reset itself meaning the truck lost total power and cut right back on. So apparently the whole truck is losing power for a split second. It does not do this at idle or slow speeds only moving down the highway.
1994 Chevelot 1500 Pickup
176,000 miles