There was a well known issue a while back - a batch of Sony CCDs were found to be faulty and suffer from sensor failure. Good news - it's a known problem with your model an it's free fix, even out of warranty!
Bad news, it doesn't sound quite like your problem - the affected CCDs often displayed purple/green streaks...
Have a read here:
http://www.imaging-resource.com/NEWS/1130908577.html
Oh rubbish, I had an A1 years ago that I practically gave away because it was exhibiting those exact problems - I simply thought it had had it. Oh well....