Mine was doing that for a while early this summer just before the throttle cable broke.
I would suggest you lube the cable immediately, and check the housing for kinks while you're at it. If that doesn't immediately fix the problem, replace the cable. (Probably the pull cable, but might as well replace the push cable while you are at it)
There certainly COULD be other problems, and you can easily check the carb linkages for binding while you're in there, but a throttle cable in need of lubrication is the most likely cause, and if it's been that way for a while, the cable and/or housing might have suffered friction damage.