Yogurt from boiled milk?

I've been making yogurt using yogurt as a starter for a while now. When I started making it I'd heat the milk on the stove using a candy thermometer and being careful not to let it boil, but I always ended up with burnt milk on the bottom of the pot anyway. So I experimented with heating the milk in the MW in stages until I got it to 180 F, no burnt milk this way and I liked the consistency of the yogurt too.
Last night I was making a batch and for some reason even using the correct time in the MW I think the milk got to boiling point. The yogurt this morning has a lousy texture, kind of gummy?and the whey takes ages to strain, it's not sour enough like the previous times.
Could all this be due to the boiling?