Try adding "/noredirect" at the end of the link, similar to this:
http://rubyforge.org/frs/download.php/69039/ruby-1.9.1-p378-i386-mingw32.7z/noredirect
http://rubyforge.org/frs/download.php/69035/rubyinstaller-1.9.1-p378-rc2.exe/noredirect
Other people have experienced this issue and the RubyForge folks have updated their FAQ. Last I heard they didn't know exactly what the problem was.
http://rubyforge.org/docman/view.php/5/460/faq.html#slowdownloads
Any of this help?
Jon