Do i need a HDMI cable to put my screen resolutin on 720p? because its not letting me do it without one.
Sorry for the late reply, my collectors edition arrived and ive only just surfaced to eat/pee.
Yes.. you will need a HDMI cable

which was why i asked in my first post. IF your not using a HDMI cable the PS3 is stuck at 580 due to the auto-detect of your scart cable. Unfortunatly to get any HD picture you do need a HDMI cable. If you have a HD capable TV then its probably worth going out and getting a cable, Dont get the expensive ones though, people will post crap about different price cables being different quality, its 99% of the time a complete farce when your not using high grade "cinema quality" equipment. So a £14 cable is suffice. This should solve your issue.
Will that really fix it? What about on an SDTV?
No.. not on an SDTV unfortunatly. Tbh.. thats bethesdas problem, theyve set the game up for HD with little SDTV support.
Think I might know the problem. This started happening to me when I manually turned off everything but 1080p on my PS3's display settings. Before, when I had 480p, 720i,720p,1080i,and 1080p checked, it was fine.
Yes this also can be an issue, however OP just mentioned the lack of a HD cable, so none of these options would run even if they were activated. However if youve turned them off your actually telling the PS3 NOT to downgrade/upgrade the signal to the resolution set by the game. So when you see the game setting itself to 720, when running on a 1080p screen, if you have it unchecked it wont do it, OR the game will try and the PS3 will upgrade the signal. This leads to issues and the PS3 reverts back to its basic default "i will run on any screen" resolution.
Tbh anyone with a 1080p tv, might as well just leave it all checked as the TV can run any of the resolutions before it. Most PS3 games generally downscale to 720 anyway.