Mine is having the same problems... on a 400k contract that needs a specific pms color: pantone 306 C, ripped through HP permanent gloss adhesive profile. We've tried just about everything; manipulating the curing and drying times, vacuum, substrate advance calibration, turning on unidirectional, enabling extra PH clean, cleaning the print heads, changing the print heads, exporting as different file types (.eps, .psd, .pdf...), even re-designing it in PS and printing from that file. We've tried printing blocks of all the cyan colors around that pms color as well, 638 C, 801 C and process cyan, and the same thing happens. Huge vertical bands of faded cyan in the same pattern. HP division is actually getting involved in this, and get this: they're having the same problem on their printer! But not a 6100... so I'm guessing at this point that, for whatever reason, the L25500 can't handle this spectrum of color.
If/when we get an answer I'll try to remember to post it. Good luck.