This fault sounds to me to be the well known dodgy CCD issue. There is a very long thread on the forum about the Canon cams that had this issue. It is caused by a large batch of CCD sensors supplied by Sony to many camcorder & digicam makers (so not just Sony cams affected).
Do a search of this forum for loads more info on the CCD issue, plus a search of the Sony web site as you should be able to get a free repair (assuming it is this issue). The free repair offer is/was a time limited offer and may have expired, but it is worth checking out.
I've just had a look on the Sony site
Here and your camcorder is listed as one of the effected models. So get in contect with Sony and you should get a free repair (fingers crossed)
Mark.