I've got one of these machines, I've had that problem but only occasionally and it always fixed itself after a restart.
If your vacuum system is holding steady, then you're fine there. Check that your vacuum is between 4 and 8 inches on the screen. It might be that vacuum sensor going bad, I replaced mine after it started going intermittent and I haven't seen that problem since, but causation does not imply correlation. If the vacuum is toooooo high then ink won't spray, but it will still purge. Check for vacuum leaks on the carriage, and in the vacuum system itself. All of the parts that make up that vacuum pump system can be sourced from the internet by searching the part numbers on them. There is a small brass nipple in the vacuum system (in that box above the blue vacuum reservoir) and it can get clogged up and cause goofy vacuum issues. It's got a clear plastic diaphragm in the nipple, with a pin hole in it. Hold it up to the light and make sure you can see light coming through that pin hole. If not, just blow it out or push a needle through it. I believe I got the vacuum sensor from an electronics place, and one of the vacuum switches from a hot tub store. Funny stuff..
Pull the carriage cover and make sure none of the clear vacuum tubes coming off the heads are stained with ink, that would mean it's drawing ink up in to the vacuum system and that's no good and would indicate vacuum is too high.
You might clean your encoder strip if you haven't, it would probably throw an error if there was a problem with it but who knows. I wipe it off daily along with every other part of the machine since it tends to collect dust and overspray.
You might check that all your connections are secure. Something could have shaken loose. I'm talking about the printhead ribbon cables, and the data cable that runs to the carriage. That kind of stuff.
There's no capping station on that machine, so don't worry about that. It's not an ink leak.
If I have an epiphany I'll let you know.
