loss of power

I have a 84 S10 4x4 with 2.8 that at times just loses power. Driving along and all of a sudden it just starts slowing down,, if you let it sit for awhile it's back to normal. This use to happen infrequently, maybe once every two or three months. It finally started getting more frequent, a much as twice a week.Last Friday it did it again and this time it won't go away. It starts and idles fine and everything is OK until I get between 35 and 45 mph then it just stops accelerating. If it happens on any kind of hill the slowdown is even worse. Any help would be greatly appreciated.