I have a 99 Ram 1500 with a lift. I'm having trouble with the tranny going in and out of overdrive at around 1800 RPM. Not always, and mostly when it's hot and under load. The RPM's surge to about 2100, then drop back down. I have not calibrated the speedo, and was told this could be a factor. The computer and governor switch are fine, and no codes have been given.

Any thoughts??