I also had this experience recently, after six months of relatively trouble-free operation. The print head carriage would stop, then wiggle back & forth a few times, then stop, and finally printing would resume. The whole thing substantially lengthened print times. I finally noticed a few bands across the width of the print where the gloss enhancer was not applied, visible in glancing light. Doing a head-cleaning cycle on the grey/gloss enhancer print head helped slightly. The printer utility indicated all was well. I finally replaced the grey/gloss enhancer print head, and now all is well. The banding is gone, and the speed is back to normal.
I suspect (but certainly can't prove) that HP's mapping/substitution algorithm for failed print head nozzles gradually slows down printing, and eventually impinges on quality as the output of failed nozzles can't be covered by substitutes. It would be nice if the printer reported how many of the 1,056 nozzles per print head have failed, so you could make a rational decision on when to replace the relatively cheap (~$50) head rather than waiting for print times to stretch into hours.
This is quite correct. It can also be the Optical drop detector becoming dirty. Also watch the GE head for remapped nozzles as it too can slow down the printer as frequent checks can be required.