Martin,
It's coming up $99 for me too. I have heard some web sites were looking at variable pricing based on your browser and cookie history. One buyer who was a frequent customer might see one price, and another buyer who just visited the site for the first time might see another. Heard of it, but never seen it. This may be what we are seeing on that item? Just a thought.
And I'm not a fan of that concept either. It can lead to one person getting a deal, but whoever he shares the deal with not seeing the same price (which appears to be what's going on here?). Ultimately this sort of pricing will loose volume sales and cost more in lost business as someone who is on the fence about an item would bite at the lower price, but might not at the higher one.
As to the Northern sale and another lower price before Christmas, I don't know. They run them in the sale circulars around here for $99 every couple of months so that may be the lowest price since the $89 may no longer be valid. But $79 final price with that coupon is still a good deal (just buy $11 more stuff to get you over $100 for the coupon).