Try reseating all connections. Possible ground issue as well. Let's start with the easy.

I've only ever seen this on a WPC95 game. On that boardset there are a couple capacitors that get changed out.

Do you have any other DMDs you can swap in from another game? Or maybe a different WPC display driver board you could put in this HS2 to isolate the problem to either the display or the driver.

Make sure boards are screwed down to back. It's probably interference thus look for ground issues. Check the entire ground grid.

Run the ram test in diagnostics

First, run display test as mschonbrun suggests, above.

Second, if you have another WPC game (not WPC-95), swap DMD controllers to see if the problem follows. You can't damage the game doing this.

If the problem follows the board, there are two filter caps on the board that are 150uf/160V. Replace them with the more common 220uf/160V caps. It's a pretty easy replacement if you have any skills at all...and a decent iron.
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Chris Hibler - CARGPB #31
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http://www.PinWiki.com - The Place to go for Pinball Repair Info

Yes.

Then the DMD is bad (unlikely causing this issue).

That's a controller/HV board issue and not a DMD issue. You can swap the DMD driver board from HS2 into FT and I'll pretty much guarantee FT will have the issue.

I doubt reseating this ribbon cable will help but try it anyway. It is a source of many animation issues that are not voltage related.Ribbon cable reseating (resized).jpg

As Chris says...95% its the 2 large axial caps on the HV dmd controller board. . Either way they are old and need to be replaced anyways.
Easy to swap out with a temp controlled solder iron.

Not sure if you meant that you can't find them on the board or that you can't find them for sale. Anyways, here's the location for both.

https://www.greatplainselectronics.com/proddetail.asp?prod=CEA-220uF-160V-RMD

wpc_dot_matrix_controller (resized).jpg