DFU Mode Won't Work

Hi again,

I am trying to get this iPhone 3gs into DFU mode. I had it going in before with a white screen, but I needed to replace the battery to get ride of an error. Went in, replaced battery. Now it works off the charger. The issue is now, if I go to hard reboot, the screen turns off after five seconds, then goes back on after being off for a few seconds. while I"m still holding home/power buttons. I don't know why, but it seems like its powering off then back on while I"m holding the buttons.

Can anyone help with why its doing this?

Thanks