Please help! Vertical dots.

I hope someone can help me as I've tried most fixes but I'm still having problems.

I'm getting small patches of vertical dots on my printouts using my Sawgrass SG400 printer using A-Sub sublimation paper (also tried Sawgrass TruePix Classic with the same results). Please see attached close up of the marks. They are about 2-3mm wide and seem to appear in the same spot on the paper and mainly after printing with black areas.

My first port of calls was the Conde Systems YouTube channel and I followed their directions to clean the capping station, the feed belt, encoder strip and encoder wheel, all to no avail. Up until today I had been using this printer for a couple of years without any (unsolvable!) problems.

Does anyone recognise these marks and if so, what do I need to do to fix them, please?

I've done some more searching on this subject and the suspect now appears to be dirty/clogged pinwheels. Thing is, I've opened up the SG-400 printer but I can't seem to be able to see any pinwheels visible. Does the SG-400 have these, and if so are they accessible?