178,000KM 2008 F250.

I changed my filters about 2 months ago no problem, drained the W/F sep this week. It's been -35/-40 here all week and I have been doing more idling then I like.

Yesterday I was driving home went into a regen then stopped at a light. My RPM was pulsing from 500 - 600 up and down. Exact same scenario happened again today...it's -10 now so not bad at all.

Filters are tight (motorcraft), quality fuel, motorcraft cetane every other tank.

Any ideas?
Thanks