Speedo and Hypertech?(not the bathing suit)

[H2 Doberman]

Has anyone had any problems with their speedometer and hypertech programmer. I had to recal. for the 38 inch tires and the mph will be off sometimes. It will be 20 +/- mph off. If I turn the truck off and let it sit and start it back up it will read fine. The truck is on 03 model if that helps any. For the most part it works fine all the time but every now and then it will be off. It is annoying because I don't know why it is doing it. Any help would be great!