Static. We have the same probs on both our hp 5500 printers. it fires the heads, especially on nice vectors on a white background. Mostly on roll up material. It can be sorted by making sure ur humidity level is above 40%. We use quite a large humidifyer as its a large area to control. But a few people i know who are working in a small area have said a simple 100quid joby has solved all their problems. Before this we earthed the chassis, put a damp spondge in, slowed the head speed etc…. not of themn were 100%. we also wipe all plastic rollers etc with antistatic spray & wipes etc. hope this sorts u out. took us a couple of months to sort out & found this out at IPEX talking to one of the larger print suppliers that sell massive spray mist machines to the large litho industry.