Elarock,
I have a 2008 2575 QXi, and my trim gauge is "special" as well. I wouldn't describe it as working intermittently, but more as being inaccurate, where the guage does not truely reflect the position of the motor. I too have checked everything, as well as having a dealer check, and nothing was found.
As a workaround/correction, as soon as we put the boat in the water, we trim the motor all the way up, or all the way down, and then the gauge "syncs up" with the true position of the motor, and we're good to go; for that outing...
I have a theory, as to the cause. Our boat is kept in our storage building, and taken to/from the lake each time we use it. When it's parked and secured, I trim the motor down, using the trim button located on/near the motor. It's my theory, that is when the trim guage and the actual motor position are getting out-of-sync. I haven't takien the time to prove this, but I will this weekend, now that someone else may benefit from the theory/test/research... stay tuned...