I'll note even some potential problem causes that might sound stupid, but in my experience, even a quaified mechanic can sometimes make a mistake, and not take something trivial into consideration, so not wanting to insult anyone's intelligence by stating some "stupid", or "I obviously checked that" potential problems, it is what it is: a simple troubleshooting list of the top of my head, based on the previous experience:
Numbers 2) and 5) are what I'd first suspect. But I woul check everything, especially when there is a problem.
Is the hub assembled properly, with all the parts in place?
Is the wheel fixed properly within the dropouts - no movement?
Geared hubs have special washers that prevent the hub from turning even slightly - if these are not seated and tightened properly, a small, one degree hub rotation of the hub (under chain tension when pedalling) could increase the cable tension). That, with most other stuff is nicely explained here: Sheldon Brown's page about geared hub setup and maintenance.
Are the bars and the shifter lever fixed properly?
Housing used should be for shifters, not brake housing - brake housing changes a bit more in legth as the cable running through it changes tension.
(pictures and explanation of the shifter/brake housing differences)
If the shifter cable passes beneath the bottom bracket, it should pass through a guide, not along the metal of the frame's bottom - if it doesn't, flexing of the frame can move the cable sideways, changing its tension.
Routing the housing should be done properly - making it as short as possible, but without too many total direction change (extreme example is making a full circle, even with a gradual radius - it will create a lot of friction and setting up the cable tension would be difficult, as well as shifter operation).
Frayed cable, or dirty/damaged housing might cause the cable to stick.
Are you accidentally moving the cable adjuster? :)
Some have a locknut, most don't. A simple marker could be a way to see if it's been moved over time. Movement from road vibes is not likely, since the barrels usually have to overcome the barrel spring tension in order to be moved over small "grooves" - most have that sort of mechanism. Even if they don't, bolts tend to loosen through road vibes - moving towards lower tension, which in this case would mean barrel adjuster turning inwards, loosening cable tension over time.