[Q] Bootloop? topic

I installed the 5.0.2 ROM and wnated to go back to my old rom, but it's not working very well, the phone starts up and then shows the google logo (from the 5.0.2 rom) and the restarts, i've tried to go back to the 5.0.2 with no success. I've even managed to flash a new boot.img file on the phone but it still dosn't work, any help would be gratly apreciated!