I have an 05 Sierra 1500 basic truck with not much in the options dept. 6cyl. Auto transmission. After rolling along on the highway and going down long stretches of downhill highway with my foot completely off the gas when the vehicle once again begins to go slightly uphill and I begin to loose speed I apply the gas pedal once again and occasionally the truck will run up in rpm like the transmission is slipping and it will loose power. A couple of things seem to remedy this. If my foot is off the gas long enough for the rpm's to drop below 14-1500 or so it seems to be OK and will recover once I start the uphill again. Also if I put the truck in neutral and then back into drive all is fine. Other than this the truck runs strong with not even a trace of transmission problems. Very puzzling. Thanks in advance for your replies.