Welcome to the forum!
From my reading on the web, you can set the output on the PS3 to 720p. I'd recommend giving that a try to see if that works any better for you. There were some comments that a 1080i output to a TV that couldn't do 1080i would be downconverted to 480 by the PS3, but those were from early 2007 and I don't know if that's true for the more recent PS3s.