My new sofrider has an odd problem. The chain keeps falling off of the front sprocket. Mine is an 8 speed, so there is no front derailleur, (which at least makes it easier to put the chain back on). I've had the bike for about a week, and on every ride I've taken the chain has fallen off at least once. It seems to be tied in with shifting, but I've got the wheel cluster derailleur adjusted so that it shifts perfectly. What could be the problem? Is the spring on the tensioning arm adjusted wrong or too weak? Short of tossing out the gearing and installing an internal geared hub, what can be done to fix it? Any advice would be appreciated.