There are several things to try.
a) How many channels do you have loaded??
More than about 15 or 20 channels and the HD
box could have "odd" behavior including spontaneous
reboots. Each loaded channel consumes memory.
Linux (internal O/S of the Roku box) doesn't tolerate low
memory situations well.
b) Try unplugging the Roku box, then hold in the
reset button, wait 15 seconds, and plug it back in.
Do not release the reset button but keep holding it
in for a full 40 seconds after plugging it in.
c) If that doesn't do it, I've had success plugging
in the Roku box to a hard-wired ethernet
connection and then updating the Roku firmware.
This fixed (I kid you not) my Roku box once when the
wireless connection went out to lunch.
d) If still no joy, you may be a victim of a dying power
supply. The wireless adapter is a separate
component in the Roku box that hangs off an internal
USB connection. Low amperage from the power supply
could conceivably cause the Roku to act a bit wonky.
No matter where you go, there you are.