Perform a hard reset on your device first, this is to ensure the device is completely "cleaned" before flashing any new firmware.
After hard-reset, login to the device, straight go to firmware upgrade page, get the stable or latest DD-WRT build file to flash on your device
DO NOT touch anything until it's fully completed when you can see the DD-WRT screen !!
Sometimes, if it show 100% completed and nothing happen for few minutes, then you may enter the DD-WRT default IP 192.168.1.1
If you see this screen, congratulation you are half way done !
DO NOT do anything yet, perform another hard reset procedure before continue.