Honda issued a Service Bulletin back in late 2006 to address issues with the Roto-Stop lever, sometimes causing it to release unintentionally. Your serial number
is in the affected range, and would probably benefit from installing the updated parts. Honda replaced these during the warranty period, but this mower is just over 8 years old now. If the problem persists, suggest you let a dealer take a look, and they will probably confirm the Roto-Stop control should be replaced. The new part number is 75180-VL0-B03, and it lists for about $25. Send me a PM with your email, and I can send you a copy of the bulletin as an FYI. It has nicely detailed instructions on how to R&R the Roto-Stop lever...
You can find a Honda dealer at this link:
Find A Honda Dealer
Finally, if you would include your name and address in the PM, I will update the registration database here at Honda to show you as the new owner. Honda asks for this in the unlikely event there is ever a product safety recall and needs to contact you.