I have this too
If it helps, I have to problem too.
I have rooted my phone and installed ClockworkMod Recover v18.104.22.168 but have not installed any custom ROM or other changes (that I know of).
I periodically get a messge an update has been downloaded, but when I attempt to install it, ClockworkMod informs me:
E: failed to verify whole-file signature
E: signature verification failed
Since I don't know what it is, I abort the install, but I can't tell what is happening. I would also appreciate any help.