Its banding, big very well known flaw on the Versant range.
If cyan is the predominant colour you'll see it about 30mm from lead edge, if magenta about 100mm past this, then yellow 100mm again and then black is about 30mm from tail edge. Basically same distance and order as your drums are.
I generally only see it on heavier weights.
Workarounds i've found are -
Try running as 150dot
Switch paper type to Cast Coated
Where possible adjust the cymk mix to have a more even breakdown, mainly helps with greys as easier to do and not always possible with colours.
A real pain but on really low volume, only run one sheet at a time.
Adjust your layout, can you position the files better on sheet to avoid.
The problem has been around since the V80 which we have, they've tried uprading motors etc on the 180 and 280 but have never solved it.
Amazing really, such a critical flaw.