Cambridge L-Nav problem -- runaway vario

I have a nagging, recurring problem with my trusty L-Nav. Occasionally, when I power the unit on, the lift tone sounds constantly, and the vario display is pegged at maximum lift (10 kts). It's clear that the problem is with the L-Nav, and not the electric vario display, as the tone continues to sound once the electric vario display is unplugged, and the problem continues once the vario is plugged back in.

Cycling power once or twice will sometimes, but not always, make the problem go away -- until the next time. Last Friday, the problem persisted for two hours, finally disappearing as mysteriously as it came. Once the L-Nav powers on correctly, it has always stayed operational for my entire flight.

Has anyone else experienced this, and found a solution?

Thanks,

Jordan