Not having a complete model and serial number kind of hampers the quality of opinions you will get with your posting.
As I answered in the [hijacked] Raymond thread, there are 2 learn procedures, one of which requires a laptop connection.
I am not sure how many 'ground' connections there are.
Does this slow down happen when sitting still or only when driving? there is a 'known symptom' that happens when the wrong vcm gets installed (one for a 25k capacity will not work in an 15k, and a pneumatic will not work in a cushion truck), where the speed sensor does not match the the math for the number of pulses it ought to have at a given RPM, that is very much like what you describe, but I am pretty sure it only happens while driving, but might be happening just if the wire on the sensor gets knocked off..