2003 B4000 coughing in the snow

Hi folks, my first post my dad and I are at the end of our ropes, hope someone can shed some light on this one.

My dad has a 2003 B4000 with a 5 speed manual. For the past few years now it's started acting really odd when driving in blowing snow, mostly on the highway only - or at least at speeds of 60kmph+. If you're following someone who is kicking up snow from the highway or just driving on a fairly snow covered road or when it's just snowing period, it will.... cough for lack of a better way to put it. It jerks fairly violently almost like the motor stops completely for a second and then goes back the way it was. I watched the gauges today as we met an oncoming semi and no lights flicker, the tach and speedo don't move it just hiccups. Not a gentle loss of power and a slow return of power, a sudden jerk. It only happens in blowing snow, strong cold winds are fine, just blowing snow will do it. And it doesn't seem to only be snow hitting the front of the truck, it will do it from kicking up snow that's driven over as well.

Any ideas at all for what can be causing this? He's taken it to Mazda and a few other mechs but everyone has been at a loss so far.