"The DP reading fluctuates wildly"
Sensor or sensor wiring faulty?
"Is 700 km further than usual between regens"
Depends on driving conditions. 300 + I'd say.
"How does the
ECU decide when to regen, and how long to keep it going?"
I guess it is based on DPF_SOOT and DPF_LOAD.
DPF_SOOT is a calculated value based on various engine operating parameters.
DPF_LOAD is inferred from the differential pressure measured across the filter.
I guess the time taken will depend on how long it takes for filter temp to reach 600 degrees and on the measured differential pressure.
Check the hoses and the sensor. To check the sensor I guess you'd use a DVM and a hand operated vacuum pump. Or get another from a wrecker which is what I did to carry as a spare.
You also play around with it and learn how to test it.
I'd say the wiring is more likley to give an intermittent fault though, not sure.