I've got several foretrex 201s and I have noticed that the connector for the serial cable on the back of the cradle doesn't work all the time. I have a couple cradles that won't work at all, in fact.
That's my first guess. I leave one cradle hooked up to my computer and it always works.
Next time it fails try fiddling with the connector back there. You might think about getting a new one from Garmin if that's the problem.