Seat belt switch bad, I'd bet on it.    I've got one of the very last L170's made, bought new 3 years ago as a hold over machine, and I've got the same problem.   When it happens, I can usually get it on the third or fourth click, so try taking seat belt off and on a bunch of times and it might come out of it.    Mine goes in streaks, about the time I'm fed up enough to replace it, it doesn't do it for a while.   It's like $180 because NH makes you buy the switch, the latch, and belt webbing.