Random iOS Mail App Freezes?
Mine did this a couple times - I rebooted it (and left it off for about 15 minutes, although this doesn't make a difference in the real world I don't think) and the issue went away - although I presume you've tried this.

I'd try properly restoring the software back to non-jailbroken state (so you have fresh software entirely) and then (if necessary) re-jailbreaking it. Otherwise, depending on your model, restore back to vanilla setup and take it into the Apple store.