Custom firmware & error 1600

[alxndr33]

I've been attempting to jailbreak my iphone 3g for the last several days most unsuccessfully. I had originally tried to jailbreak on firmware 4.2.1 but kept getting error 1600 (I used pwnage to create a custom firmware). After that didn't work, I decided to downgrade to 3.1.2 and jailbreak (I had done this before). Unfortunately, I ran in to error 1015. So I looked around downloaded tinyumbrella and managed to successfully jailbreak on 3.1.2. However, most of the cydia apps I'm interested do not support that firmware. So from my 3.1.2 firmware I went into DFU mode and tried to update to the custom firmware 4.1.2. But, again, I ran into error 1600. There seems to be no way around this. I have read there is a possible fix with iReb but, I don't have a PC so that won't help me. Does anyone have any tips to help me workaround this problem and have a jailbroken iphone running on 4.1.2? Thanks.