A "cheaper but new" PSU could be ultimately dangerous to your system. Read the assorted reviews of cheap PSUs on jonnyguru.com (or hardwaresecrets.com, or hardocp.com, or xbitlabs.com, or... .)
Post your specs if you'd like assistance with getting the right size, but for a single graphics card up to a 4870, a 500W Antec Earthwatts is sufficient. Those are only $70 now on Newegg.
got the new psu was a jeantech £50 worth same issue so did some more reaserch on the internets regarding this issue and apparently its a problem caused by the BIOS chip breaking the only soloution is ot RMA the motherboard or contact asus to send you out a replacement.
mine is out of warrenty and i can't be waiting up to two weeks for a replacemen bios so i got a new motherboarod all working fine.