If you do a search on HEQ5Pro here you'll find some threads with conversations amoung those of us that suffered through some early problems. They seem to come down to problems with handcontroller, cable, or power supply. Not to say those are the only possibilities of course.
Mine was a dodgey connection on my cable. A normal cat5 or cat6 network cable can be used in place of the coiled springy black cable if you want to eliminate this possibility (I find the 5m cable I bought handy sometimes if I want to get get some distance from my setup anyway).
Power seems unlikely if its only one button with problems. Handcontroller sounds like a distinct possibility - might need a trip back to supplier if still an option.
(my supplier happily replaced my cable once I'd figured out what was going on - slewing and alignment would just drop out randomly anytime for me before that

)