On your system menu (second icon on the left) there is a video display setting.
This setting tells the PS3 what the highest resolution it should use is. Once you change this setting to 1080i the PS3 will use it when it is playing something that has 1080i resolution.How do I Change my PS3's video settings to HDMI?
You have to find the channel for the HD port on your TV and you have to start your PS3 I think!
Go to settings/video or output settings i think, and there you can choose whether to use 1080p/1080i/720p.........there are about 6 different scaling options you can include or exclude whichever you want. in your case I would exclude the 1080p one it could be the problem. have a look around there I am sure you will get it sorted.