Hi arnott and welcome to the Q&A Forum!
Boy this problem is a stumper! Usually when a sensor goes it affects all photos. The inside looks fine but the outside shot looks like it has a sensor problem because of the lines as well as significant washed-out exposure.
I've peeked at the EXIF data for each photo, and looked at the largest sized image in you Flickr gallery. Everything appears to be fine as far as settings. You even used minus 2 exposure compensation for the outside shot, which should have prevented over-exposure.
There are really only two things I can advise at this point. The first may not even help but it's worth a shot. Do a low-level format of your memory card.
If that doesn't help. I think the only route to take is to contact Canon tech support. They can confirm if it's a defective sensor. If it is a defective sensor and the cost to repair is not worth it, ask about their Customer Loyalty program. They usually give a discount on another camera should you want to get the Canon brand.
I owned the S2 and simply loved it.
Please let us know what happens. Good luck!!
Related reading:
Canon Customer Loyalty program:http://www.digicamhelp.com/camera-logs/cool-stuff/canon-customer-loyalty/Exposure compensationhttp://www.digicamhelp.com/camera-features/advanced-settings/ec/EXIF datahttp://www.digicamhelp.com/glossary/exif-data/