Acceleration problem

I have a 2001 Sonoma 4.3L Vortec with 37,000 miles. When I start the truck in the morning, I'll sit for a few minutes to warm the engine a little. I put the truck in reverse, pull out of the driveway, shift to drive and when applying gas the engine revs up, RPMs don't go up, without the truck going any faster. As soon as I turn the corner, 2 houses down, the revving stops and the truck runs fine, I don't have any other problems with it the whole day. When stopping at a light or stop sign, the truck has no problem accelerating. Any ideas? There is no service engine light that comes on or anything. I've checked trans. fluid levels and they are fine. Could it be that the trans. is still cold?

Another quick question, should the fan always be spinning?

Thanks!

Do you have a clutch fan or electric?My clutch fan makes the truck sound like its reving alot but its just the fan,sounds like it might be your problem.

It's an automatic, so it would be the electric fan. Is it always supposed to be running? It never seems to stop