After getting off the highway and after coasting on the ramp, then beginning to accelerate the RPMs go up on the gauge but nothing really happens in means of power until the gauge reads 1200 RPM. The first time this happened was coming down a very steep hill and coasting up to a stop sign and then slowly accelerating out where the same problem as previous happened. The first time this happened it went on for two days max. Now on the rare occasion it does happen (only happened a handful of times) it only lasts until the truck is shut off. Don't know what to think?