First of all, it looks like that you use wrong dithering. With thermal transfer printers, you have to use "Angled Screen". If you have set the Dithering to angled screen and get this results, it could be that you have set a wrong (too high) value for lines per inch. Or the media is not suitable for the PC600.
The picture where the upper parts of the bands are different to the lower parts looks like that the head is not correctly adjusted, but it could be a problem of the media too. The streak I see in picture 4 could be a problem with ribbon wrinkling, what could be a result of a wrong adjusted head too. An other reason for that issues can be that the media is not suitable for thermal transfer.
The banding you see all over the pictures is type of construktion and can't be avoided. It is not possible to print banding-free with this machines except the pictures are only as wide as the ribbons. Specially because this machines are driven with stepping motors, as soon as they are old and worn out, there will be no possibility anymore to get good printing quality.
As soon as you change dithering you will see that this printers are not really suitable for picture- or photo-printing.
If you print mostly pictures of 2.5 x 3.5", and you really need to use a thermal transfer-machine, you should think about a Summa DC3/DC4sx. The price of the consumables are much lower, the pictures will fit in one width of the ribbons and will not generate banding, and if printing larger the feed of the media is much more reliable.
When still testing, think about that the PC600 is really outdated. I would suggest to don't spend to much time to make this machine suitable for you (if possible).
