ODD problem-motor revs, rpms stay same

[Gregs71]

Ok, today we were driving back from the storage unit at about 55mph with a truck bed full of odd and ends and pulling a small harbor freight trailer total weight maybe 1000 lbs and its about 100 degrees outside.

About half way up a slight grade in the road the motor instantly reved up as if I had pressed the accelerator. It stayed like this for 20-30 seconds, then returned to normal. I hit another small hill and it did it again. This time I accelerated after it reved up and I was able to speed up without any apparent issues (which to me would rule out tranny).

THE WEIRD THING... the second time it happened (didn't notice the first time) I was watching the tach and it didn't seem to move despite the motor getting louder??? The tach did go up when I would step on the accelerator.

I had thought about torque converter, tranny, but the tach not changing has me confused. Tranny fluid is topped off and there is no discolor to it.

Any ideas???