How to figure out what's causing a jailbroken iPhone to crash?
Solution 1:
The first thing that I would do is to check in the iPhones Diagnostics reports. In Preferences go to General>About>Diagnostics & Usage. There might be information about your crash there. Your other option is to install Crash Reporter and syslogd from Cydia. You would be able to get some information about crashes there. If you're having trouble even getting to Cydia, try disabling MobileSubstrate, those plugins tend to cause a lot of crashes on jailbroken devices.