I have a Dahon D7 and for the past 1000 miles I find myself constantly having to adjusting the rear shifter barrel that is near the right handlebar. I keep rotating it clockwise or counter clockwise to set it. But then after a few miles of shifting it seems to unwind. Do I need some kind of loctite on those threads?
I'm not sure. I found the same thing on my Helios - the LBS that I bought it from was constantly fiddling with it for me because the low gears kept getting out of whack. That bike was stolen, so I can't tell you if there's a permanent solution.
The cable might be sticking inside the cable housing and perhaps exacerbated by the folding process. Try taking out the cable, wiping it clean and apply a coat of grease, while spritzing the housing's inside with a lubricant. You may want to change the cable/housing altogether, just be sure to get a quality brand and follow the original cabling route.
If this doesn't work, perhaps your rear derailleur is out of whack and may need servicing (cleaning, lubricating, alignment).