Well, You've all had the same ideas we have and I appreciate it greatly. Thank you.
The reason I came to this forum is to see if there might be something we're missing or if anyone else has had a similiar problem that the engineering or maint departments don't want to admit. The anonymity here can be a great advantage.
Ours are all -800's and it happens on a number of different planes. In one case we did find a chafing wire bundle and there's a service bulletin out on that now which we're actively employing.
The trips seem to all reset after a short time which indicates a temp trip. My personal theory is that since there's no anticipation in the system like in the DAC planes, when the engine comes to an abrupt idle after high altitude cruising, the 9th stage opens up to compensate for the sudden lack of air and brings with it hotter air tripping the 490. Once things stabilize the system resets just fine. That's a view shared with some of my friends up north who, for professional reasons, prefer to remain anonymous.
I have heard from one of my old comrades-in-arms at a competitor who tells me that three years ago, before Boeing changed the software raising the flight idle RPM, they were instructed to pull back to 50%, let the things stabilize, then pull the rest of the way back to idle (these guys don't have autothrottle). Everyone else I've talked to tells me that when using the autothrottle there's no problems since the software upgrade.
We do have one chronic bad actor and we've even changed the engine, (for unrelated reasons) all the valves (because it kept tripping), plus the wiring harnesses, and it still trips. I want to go after the precooler but that's just my own personal brand of kneejerk shotgun troubleshooting.
My other idea is to see if it's tied to one group who may simply be pulling the throttles back like some used to do in DC-8's to see how many noise complaints could be logged under the approach path into ATL on a cold winter's morn. (Nothing can backfire like a short duct JT-3)
Once again, thanks all very much for the input. Keep it coming...