It's an RF keyboard so the first thing I'd do is make sure that no additional sources for interference have been added. Cordless phones are a biggie, and I'm sure cell phones, game controllers (ala Xbox 360, Wii, PS3, etc) would rank up there as well. Changing the channel on the keyboard might help. Since the behavior is inconsistent this is most likely the cause.
If everything else checks out, the RF transmitter or receiver might be screwy. I'd look to see if there's some sort of manufacturer's warranty available. A lot of keyboards and mice from good brands (i.e. Microsoft, Logitech, Kensington, etc) have warranties on them, whether they be 90-day or a year.