try pulling out and reseating all boards. I would have to say you might need a new duplex card.

try swapping the top/bottom and intermediate cards with #2 car and see if problem moves to #2.

I would check out the 24 volt power supply that supplies 24D and 24C if this voltage gets below 20 volts then you can have some strange problems. I looked at the board prints for top and bottom, and intermediate cards that control car and hall calls, the only thing that is common is the 24 volt supply, the 5 volt supply and the common, I would check the 24 supply first as this is most likely the problem. there could be a problem with the 5 volt power supply but i dont think it would give this problem, check 5volt on the power board from sc to sp test points on the front of the card.

which SolidState is this? The DMC? LMH?

If it is a DMC you can try changing the car that is the master to the slave and the slave car to the master. Both the 5 slot card system and the DMC are sensitive to volt settings make sure you 5 volt is stable, both have adjustable pots to set the voltage.

If it is a DMC you can try changing the car that is the master to the slave and the slave car to the master. Both the 5 slot card system and the DMC are sensitive to volt settings make sure you 5 volt is stable, both have adjustable pots to set the voltage.

how do i know a dmc or lmh?

normally if u ask that then you shouldn't be working on it. But it should be labeled somewhere.

Sounds like a top and bottom board. You can get a new one from Sees for around 200.