I'm also having the same exact problem.
When watching a movie, and forwarding one or two frames ... the first one is okay. As soon as you try to go to the second frame it'll like the button is being held down. The only way to stop it seems to be hitting the fast-reverse button.
Oh damn, I spoke too early... Aram is correct. The first frame is okay when you press the left or right arrow on the remote. But as soon as you go to the second frame, it suddenly goes into rewind or fast forward mode.
It never used to do this!
Roku employees, are you listening?! We need this fixed!
I emailed Roku about this problem, and they only replied with a form letter telling me to restart the Roku box. When I replied to that email, they replied again with the exact same form letter. Sheesh, this is HORRENDOUS customer service. I bought this box for the Netflix connectivity. If I knew that Roku had such horrible customer service, I never would have bought this box to begin with.