Highs and Lows - Part 2

Buzz's sugar yesterday morning was 160. You'd think that would be good right? It is good - but when it's that low, we don't give him any insulin - but yet we know that his sugar will go high within 2 hours because that's just what it does. And that's exactly what it did - went to 500 about 2 hours later. I gave him his insulin (humulin n) and I decided to stay home to do a glucose curve (again, thank goodness for being in graduate school!). Here are his sugar reports (roughly, because I don't have his diabetes book in front of me):

7:30AM - ate 1 cup of Royal Canine High Protein food
8AM - 160
11AM - 500 (gave 4 units of humulin n)
1PM - 505
3PM - 390
6PM - 200ish, ate 1 cup of food
8PM - 490 (gave 4 units of humulin n)
10PM - 450

I'm not understanding why he has these days of such extremes. For the past few weeks, when we check his sugar in the morning, it's in the mid 300s. At night, it's usually a bit higher, 400 or so. The night before all of this happened, his sugar was in the low 400s and we gave him the usual amount of insulin - so that can't be what caused it. My husband thinks we may need to try the mixed insulin (a mix of both fast and slow acting insulin), but I hear that doesn't have great results in dogs.

If you have a diabetic dog, what kind of insulin do you use? How long did it take you to find the right insulin and to get your pup regulated on that insulin? Any advice for a newbie?