Well i started having problems with cydia once i updated to 4.2.1. I think i might have installed a package and cydia did not like the combination of packages and sources that i had on my phone. Every time i would scroll down to the letter I it would crash. I tried lots of things like iFile, SSHing, and deleting sources and trying to reinstall using Terminal. The most simplest way you can rid of this problem is to download Icy and uninstall the package you crash on through that client. Sorry if it's been posted but i found this helpful.