I had something similar happen to me. Cydia would launch and then instantly crash (close). After a day to 2 of repeated JB, I gave up and did the Erase All Content and Settings command. After watching a series of failed boot attempts after the Erase, I put the iPhone in recovery mode, reinstalled the latest firmware, set it up as a new phone in iTunes and then did another JB. It's never worked better. No issues at all.