I had the same problem in Silverton CO. 9300'. I also had problems last week on the road coming home from Marysvale Ut. I took things apart and found the sparker had a 1/4" gap. I bent it to be only 1/8". Works good here but we are only 1800'. Won't be any place high until next July back near Silverton.
My water heater had a similar problem when I got it, but not just at high altitude. I also bent the sparker closer and it fixed the problem.
Don't expect the dealer to be able to fix this. If the tech's were actually knowledgeable, they would be working somewhere were they could make some money or have their own shop.
2016 Forester 2401WS MBS
Life is short...eat the cookie