Can't get good rubygems installation

I don't know how to fix this. While installing the radiant gem last night, something happened to my ruby gems installation. I've removed and reinstalled it a couple of times, to no particular effect. I'm working on Kubuntu (7.10) Linux. (I used apt-get for one install, and when that didn't work I tried the adept package manager - results were the same.)

"gem list --local" produces or "gem help" both produce -
/usr/bin/gem:23: uninitialized constant Gem::GemRunner (NameError)

Any ideas how I can get things back to normal?