I have the identical problem with the same Honda lawnmower. Apparently, the drive belt on my lawnmower may need to be replaced or fastened better (slipping). Irregardless, I plan on having it fixed/overhauled for the winter (1st time in 9 years).
Good Luck - let me know what you find out.
Patrick
howtomendit.com for free repair help, information and advice.
Try fixfault.com to fix lawn mowers.