To all those that own a 64S/100s or HP9000/hp10000
If you want your machine to run well for as long as possible, please get it serviced. I have read many articles here about heads failing and how to bring them back, if you are asking this question, your heads are already on their way out.
If you read the service manual you will see that there is a periodic service replacement for a multitude of parts, these include caps, wiper blade, wiper sponge and a few other parts, these being changed will obviously prolong the life of your heads.
Consider washing your windows with a dirty cloth, yes, pointless, then consider not having changed the wiper sponge for 24 months, ink particles, dirt, dust will build up on this and can be transfered to the heads.