iOS 6 - I erased my jailbroken iPhone 5 and it works OK, however..

[leonicht]

I bought iP5 with iOS 6.0.1 (yeah, that's what I was looking for). I've jailbroken it and then I realised proevious user did not actually erased that phone (there was still some data). Then I did this and it didn't crash. I configured new iPhone and it turns out Cydia doesn/t work. It flashes (I mean it instantly gets minimized). Furthermore it turned out booting phone doesn't work properly each time - sometimes it gets stuck when the apple logo is on the screen. Then I push both button for 8 sec. and next attempt to boot phone is OK.

-is this issue with booting a result of erasing phone running with JB?
-as cydia deosn't work should I try re-jailbreak? is it safe?