Like you, I have also had puzzling intermittent problems with the Happijacks. I recommend you read the sticky at http://www.rv.net/forum/index.cfm/fuseaction/thread/tid/22881036.cfm
I used this as my guide in totally disassembling the Happijacks, cleaning and lubricating. Water gets down inside and grease gets stiff on the screw. It is possible to take the motor unit off the top without removing the jack from the camper. I've done that and added oil (better that than water). If the problem is electrical, depending on how your camper is built it may have an ethernet connector where the Happijack controller cable passes thru to the outside. I've replaced mine 3 times in 10 years because of corrosion.
2006 GMC 2500HD 6.6 diesel
2003 Outfitter Apex 9.5