I'm gonna gamble on the resistance load being inaccurate. Tons of jobs got turned over early with improper loads. You should have 60 Ohms on your CAN channels, High to Low (Blue wire with white stripe to white wire with blue stripe).

Multi car groups, people often forgot to take off the HC1T and HC1C jumpers. I usually take them off the master car, and all slaves up to my final car, which stays on. If repeaters are used, I sometimes take the jumper off the A channel to get loading correct. Remember it's supposed to be an End Of Line resistor when you leave something on, so make sure it's actually the end of the line.

90% of my odd, intermittent, hard as hell to duplicate, CAN issues have been due to piss poor CAN loading.