Please help me out here.

I bought a iPhone 3G to help me make it until iPhone 5 after my iPhone 4 got stolen. Problem is after the guy sold it to me I let him hit 'reset all settings and clear data' in the settings after I bought it and it got stuck in recovery mode loop. Apparently what he had done was unlock and jailbreak the phone using the iPad baseband on the phone. After a very difficult time (the Apple store wouldn't help fix it, iTunes wouldn't restore it) I finally found a internet tutorial to fix it. I acutally had to restore to custom restore firmware and then came out with no signal, and had to use redsn0w to tell it to put the iPad baseband BACK on the phone AGAIN to get a cell signal from AT&T after the restore. Problem is now the phone works but the crashes and lag almost render the phone unusable. Is there any way to get this phone back to FACTORY 4.2.1 or better yet FACTORY 3.1.3? I'm currently on iPad baseband with custom 4.2.1 firmware. Also how risky would it be to try this? I don't want to break the phone and after going through so much to get it to simply start and get a signal. I don't want to risk permanent damage, but these crashes and lag are hurting the OS performance bad enough for me to at least ask. I don't need any kind of jailbreak or special apps, just a regular stock iPhone 3G with no extra RAM being used for bells whistles unlocks or jailbreaks. If anyone has detailed info on what is going on here I'd appreciate it. For the record I don't have or know anything about if the guy I bought the phone from had SSH blobs or not. I've heard that's what's blocking me from a stock restore but I don't