Ok so i was having the same problem with my iphone 4 on 4.2.8 for verizon.
I'd post the link but since im new, it wont let me. Go to youtube. How to Jailbreak Verizon iPhone 4.2.8 UNTETHERED Redsn0w
thats the title and the author is snoman6363.
My phone gave me trouble trying to restore it to factory settings, dont know why, but instead I just backed it up and erased all data and settings on the phone directly instead of through itunes.
Let me know if it works!!!