I had the very similar issue with my JP a few weeks ago.
I would power on the game, see the ROM version, then the DMD would go blank while the rest of the machine seemed to function normally.
The DMD would only sporadically update during game play.

I opened the back of the game and pulled the DMD out. Check all connections and anything else I could think of and it all looked just fine. Powered the game back on with the DMD out of the game (still connected, just face down on a mat on the playfield glass) and everything worked correctly. The DMD was showing all animations and updating correctly. Assuming it was a connection that I had somehow corrected, I put everything back together and powered the game back on.
Yea, that didn't work.

Now it was showing the ROM display, then instantly saying T-REX switch error and then going blank again. I had not even looked under the playfield at this point and my T-Rex was working just fine. I tried to access diagnostics and the DMD stayed blank. I would hear the sounds as I moved through menus, but the DMD didn't show anything. I opened the back up again and checked connections again, which again looked fine. Pulled the DMD out of the machine and powered the game on. Again, with the DMD removed but connected, it would work correctly. No T-Rex switch errors and all animations working as expected. I opened diagnostics and checked my T-Rex to be sure it operated correctly (It Did).

As GLSP3022 mentioned, I knew the ribbon cables would sometimes get pinched. Mine looked like it had a sharp bend in one location as though the DMD had been set down on top of it. I straightened that out and that was when I noticed the power connector coming to the DMD was able to move up and down a bit when it was connected. It definitely seemed too loose to be correct.

I replaced the power connector and made sure it did not move/twist up and down. As woody24 said, it should be fairly stiff and take some force to move. Now the connector won't move at all without some force. I also cleaned up the connections for the ribbon cable and re-flowed the solder for the connectors as they looked to have a few cold solder points.

These changes seemed to have fixed the issue. It plays correctly and the DMD has been working correctly since. I have not received another T-Rex switch error and I'm not exactly sure how/why that came up. I know getting the cables in the bottom of the backbox to sit nicely is a pain (thus my pinched ribbon cable).

Not sure if this will help you at all but that was my experience. This is my first Pin, so I'm still learning all the intricate details of maintenance and fixing issues like this. ChrisHibler's comments about the data not getting to the DMD make sense to me and seemed to be the issue that I was experiencing.