Just want to note that while it's best to start clean after a jailbreak it's hardly necessary. I've restored successfully using iTunes going from jailbroken 4.3.3 to 5.1.1 Stock then jailbroke and restored to that.
I did not restore my cydia tweaks as I figured those could introduce instability as many are very specific to os.
Restoring from a stock iPhone same os should work.
The big difference is in the computers everyone uses. Some are Mac, windows xp, 2000, 2003, win7, 2008 etc etc with all different settings and firewall antivirus.