Some early models of the N5 had problems with run away slewing (scope takes off during a goto and never stops) and hand control lockup, especially when using a computer to control the scope. Some have claimed that low power (weak batteries) or a loose power connector (see here for a fix) causes most run aways, but others feel it is more likely a software problem. It is known that newer versions of the software (firmware, technically) resolved the problems with computer-control related lockups as well as most run away problems.
Unfortunately, the NexStar 5/8 cannot have it's software updated by downloading new software via the RS-232 port. Instead either the hand control and possibly the fork/mount need to be returned to Celestron for upgrading. If you feel you are experiencing software related problems, you will need to contact Celestron to see what can be done.