Your LBS doesn't know anything about wireless computers and their batteries effect on the computer. If the computer batteries get weak then yes your display will dim, but if your sending unit batteries get weak this WILL NOT effect the display being dim, it will however give you a bad readout like your describing!! First change the sending unit battery to see if that cures the problem, if not, Cateye has a very good (the best I've ever seen from anyone bike related product or not) and they will take care of you quickly.

Typically (at least with the Cateye 2 that I have), the sending unit battery will last about 1 to 1 1/2 years, whereas the computer battery in mine is now 4 years old and is the original battery.