My 1999 mazda 626 4 cylinder shifts rough after 40-50mph or barely shifts at all.?

The RPM would jump too. The engine light was on. My friend read the code after plugging it and it read CODE P0708. he ERASED it. The car ran great! Shifted well and even had it near 100mph fine. The next day it is back to doing the same thing. They said its the shifting module(solenoid) or a connection may be bad that goes to the . Where is the shifting module on there? Anyone had this issue? Im sure its not the , trying to correct the matter. thanks.