Can't get into DFU mode

I'm having an odd problem that has me stumped, and am wondering if anyone here can shed some light.

I have a 3GS running 4.2.1 and 5.15.04. I want to jailbreak it with the new Redsn0w beta.

I'm fairly sure my phone has the old bootrom because the serial number says it was made in week 38, but I want to double check. I just don't remember which one it has. And I'll have to get into DFU to JB anyway.

But I can't seem to get into DFU because the phone gives me the Apple logo too fast.

All the instructions say to USB it, shut it down, and then hold down Home and Sleep for ~ten seconds, then just Home until iTunes sees the phone in recovery and the screen is black. That's great, but about ONE second after I hold both buttons, I get the apple logo, and then the phone boots.

What can be done? Am I doing it wrong? I feel like I must be, but I'm following the instructions exactly. I've done this about ten times and it just instaboots on me.

*edit* Forgot to mention it's a Mac iPhone.

 

Yes. I've tried it both ways (connected and not connected). Is iReb safe to use for this problem? I just came across it with more googling.