Random and Infrequent Dog Aggression

I moved in with my SO a little over 2.5 years ago, and I brought my then 6mo pitish/mastiff mix puppy with me. She had four other dogs at the time. The dogs were all familiar with each other, and had spent time together no problem before the move. In the first few months my dog pretty viciously attached two of the other dogs on a few separate occasions. We tried to figure out triggers, and the usual muzzle, training type stuff. And it pretty much worked.

99.9% of the time everything is fine. We crate or muzzle my dog when we are out of the house because we don't trust him. But every once in awhile, and I'm talking 6-9months in between events, he'll just randomly attack one of her dogs, and now it's pretty much always the same one, another older (but not old) male. The dog he attacks is about as passive a male dog I've ever seen. We cannot figure out what sets these off. Or what could be done to prevent it. There are serious concerns about having my dog around now. Really, concerns that he's safe for anyone/dog to be around, because it really seems totally random. The only other thing I would mention is that my dog is really really dumb. Can a dog be too stupid to realize what he's doing? At the same time, when he goes into these attacks, it's kind of like he's not there at all.

Of course, I'm writing because after being I think now almost a year without a real incident, there was a pretty viscous attack this morning that required a vet visit.

I need help.