have you removed the logic card and power dist board and looked at the diode 'drive' board?
these trucks were notorious for buring up those diode drive boards if they were ran on low battery power for too long a period. That can cause all sorts of strange drive system problems from running slow to not running at all.
You might want to take a look at that board and see if any of the diodes on it are burned. They are 1/4 watt diodes so they are fairly large and easy to tell if they are damaged from heat.