Just wanted to update this thread.
I have been working on a Fire for a customer, in NY (maybe it's the same game 7 years later?).

System came in with a Rottendog MPU that had blown transistor (the bottom one in the column next to 1J19) then someone accidently put the wrong transistor in to replace it.
We replaced the Rottendog MPU with another in the interest of time.

Once the game was up, I was encountering the same issues - you'd be playing and get a random TILT.
Not necessarily the first game, but usually within 5 games or less.
It had to be the playfield tilt for the same reasons as was stated above (no tilt warnings or sound effects, just instant tilt, and move on to next ball).

- Checked all switches in the game, saw that the playfield switch looked to have been removed at some point and was also separated quite a bit so it definitely wasn't making.

- Put the game on its own dedicated 20A power (have seen issues with random bugs on games that are run on long extension cords and on loaded circuits).

- Assumed we had a random short in a wire affecting the lamp matrix and looked through harnessing but didn't see anything obvious.

- Checked all the diodes on every switch.

- We filmed game play in slow motion and saw the fire lights turn off right after a left flipper actuation (right ramp was in the lower position, center post was raised, left ramp had just raised but it seems that raised JUST after tilt was declared - so timing thing).
We saw the lights of the 'fire' in the playfield dim at that exact time, and the center post lower at that exact time (and it had lowered early, you can tell by the frequency of the flashing light above the post when it should have lowered).

We started investigating that flipper and saw that the left flipper coil had been replaced at some point.
When it was replaced, someone did not install the two diodes needed for this coil. The schematics call for diodes here.

We put the diodes in and played.

We are about 24 games in and have not seen a single random tilt!

The best theory is, lack of diodes on the coil was injecting noise into the system.

Just wanted to update this thread incase someone else experiences random Tilt on a system 11 (or specifically a Fire).