It's gassed. This stresses the PS as well. You need to bite the bullet and replace.

Once it starts to go, the glass is done and you need to replace. Again, leave it too long and you may have to replace the PS as well, although I have seen people leave them gassed partially forever with no other issues.

I know there are greater power demands placed on the PS when the dmd starts gassing. As such, maybe when testing the verticals, there is enough power to provide more for this, but you again notice the issues when the horizontals are going.

Either way, you have other DMDs in your other machines, you can swap one in to see that it works fine, but otherwise, there isn't much else to discuss with DMDs. Once they go, there is usually next to nothing you can repair, and it's just one of those things you have to buy.