Downgrading from 2.71 SE-A

Hey,
I've installed the Custom Firmware 2.71 SE-A on my PSP, but I would like to install 3.03 OE-thing now.. The problem just is, that I don't know how to downgrade from the 2.71 SE-A to 1.50... I tryed the the Dark_Alex-downgrader, but it just stalls my psp and then reboots it after a few seconds. I also tryed to upgrade directly from 2.71 SE-A to 3.03 OE, but it did the same as the Dark_Alex-downgrader (Stalled and rebooted).
Back when I had a the 2.01 Sony Firmware installed, I just updated to Sony Firmware 2.71 and then downgraded, but it will not upgrade now due I already have some sort of 2.71-firmware installed...

Anyone know what to do?

Simply go into recovery mode.
The way I upgraded to 3.02 OE-B, is from 2.71 SE-C, but I had a .rar file with everything inside it, I can't find it anywhere on my
P.C, il continue my search.

I know how to go to recovery-menu, I think. But it is just a blue screen where I can toggle 'hide corruped icons' on and off and that movie which plays when you turn you psp on, that can be toggeled too. Also, there is a menu called Advanced, but I don't know what the commands inthere does...
What am I suppost to do inside the recovery-menu / mode? (assuming that recovery-menu and recovery mode is the same...)