I also had bad luck with a cheapo wireless computer. It seemed to be prone to interference, and would show crazy maximum and average speeds. Certain locations would set it off.
I forked out £2.08 for a wired replacement, and have no complaints (so far, at least).
It's hard to see the point of wireless bike computers. Granted, there's no wire, but at the expense of added complexity involving two batteries instead of one.