I am seeing it flag more frequently now. When viewing active excuses under the Debug menu it pops up at the end of a run right after the contactors drop out. I don't know of that's just because that's when it processes what it "saw" during the run or if it is actually related to the contactors. This is despite the LED on the 150fpm board toggling at what appears to be the proper speeds.

Kind of a gnarly problem.At this point I would replace the Relay Pilot Board. Intermittent problems like this require mostly educated guesses. The position and velocity encoders all run through the Relay Pilot. As does MSU and others. Have you checked for loose connections, or tried unplugging end re-plugging the connections to the Relay Pilot and Relay interface boards?

Speed board error right?
Something common to both up and down directions, terminal and intermediate landings, and no other faults.
The "100" part of the fault gets me. It's like the controller isn't seeing something right at the get go. Does it slowdown normal or at reduced speed?
Auxiliary contacts on M?
Bad contacts on a relay in the speed box? Those are bifurcated contacts, not just the run of the mill 4 pole relays.
Flakey hatch switch?

Disable the Door

Jump "SS to ALT"
usea a Hand Tach to monitor the Speed should run at 50% contract speed, if not run at 50% re adjust "Alt_MVEL"

re adjust the Speed board 1 to activate @ 50% and 2nd @ 90%

replaces relays in ETS board (take care 2 difrent voltage relays in same board)

Some errors don't register until the car stops. I think this is one of them.

Also, if you're going to do any speed scaling adjustments, go ahead and start with the MSU zeroing procedure. It's simple and can make a big difference.
Most of the really intermittent issues I've had like this have come down to ribbon cable connections, Molex & card plugs too but especially the ribbon cables. Except the last one was an MSU board. If you let the car cycle and gently tap plugs, move wires, flex cables, etc. sometimes you get lucky and catch it. I like to clean ALL the plug connections on Miprom 21s with De-ox-it and look in the ends of the Molex connectors for distorted/tweaked terminals. I replace the bad ones with the gold plated "Trifurcon" terminals that grab the sides of the pin too (same ones I use on Ascension Power supplies & Accu-Flite SCR plugs.

One more thing, if it were me, I would want to be able to recreate that exact fault sequence with the 100, Px Tx etc. Would simply setting the 150 fpm board incorrectly produce the same error? Or just the relay contact failing?

Thank y'all very much for the advice. Haven't heard anything back from that building since replacing the relays on the ETS board. Of course, it is Monday, so maybe they just didn't call over the weekend. I really appreciate all of the suggestions and will be updating more as I work my way through them.