Downgrade a jailbroken Iphone 3g from OS 4.0 to 3.12?

[AmandaAmmy]

I bought a Iphone 3g and it ran fast, but when i jailbroke it i also upgraded to OS 4.0 wich was a big mistake my Iphone is running crazy slow i mean it lags like crazy
How can i downgrade it without re jailbraking it?