I've seen this kind of distortion many times, not always the same root cause mind you, but I guess, probably the main culprit is damaged memory (thebit planes shift around and fail to alight correctly before writing to storage), either the chip or the storage. You might want to format your SD card first. If that doesn't work, take the batteries out of the camera for about 5 minutes and put them back in, any improvement ?
http://www.pcmag.com …=bit+plane&i=38689,00.asp
http://en.wikipedia.org/wiki/Bit-plane
If not whatever is causing it is probably a permanent thing, and it's either going to be the sensor, or the buffer memory where images go before being written to the storage card, like in burst mode. Typically this issue (on computers anyway, not cameras) is down to a corrupted file, fragments of the file getting lost from A to B, or fragmented (non-contiguous) memory. Has it been near a strong magnet of any kind ? non-shielded speaker etc ?
A factory reset (if there is such a thing on a 620) might be an option too.
Best of luck, let us know how you get on.