You might be feeling annoyed with this problem. (I do) Problem could be:
your local source cache Example: /usr/lib/ruby/gems/1.8/source_cache
; try removing this file (Your path might be different than above given; check your path first using #gem env) If this does not solves your problem
check your env var REMOTE SOURCES (#gem env)
If it is not gems.rubyforge.org then use following command to install:
gem install -r gem_name –source http://gems.rubyforge.org
If this does not solves your problem(is your internet down?)
Borrow gem files from your friend and install them without -r option.