We have had this problem on an off with a Roland 540. Went thought he hole lot: cleaning, different medias, temperature settings,colour profiles, even got a new head. Nothing.
Just a week ago (I think!) we found the 2 or 3 problems:
1.) If we are not using the printer A LOT, occasionally we get stripy prints on solids. The printer needs to run a lot and it "cleans itself" through.
2.) For sensitive solids, use uni-direction (but without the first point it does not solve the issue!)
3.) When you take out a cartridge and shake it, don't print for a couple of hours, let the ink settle.
We are printing over 100m (300feet) of the same pink colour on Phototex at the moment. The first metre was very stripy, after 4-5m of printing the stripes disappeared.
But when I took out my magenta to check how much is left in it and shook it (gently), I immediately got some stripes about a 60cm (2feet) into the print.
I so hope this helps you, we went though a lot with this and it was VERY frustrating.