Get the dog thoroughly vet checked.
Hypothyroidism and tick borne diseases, first [FULL PCR/DNA tick panel] and then checked for intermittent pain issues.
Dogs hide pain out of instinct and your comment that the older he gets, the crankier, makes me suspect increasing discomfort.
Hip dysplasia, knee issues, spine issues, even cancer [which they are prone to] could be possible causes.
Low thyroid and tick diseases also cause intermittent “sudden” snaps.
Honestly, I think your dog is sick.
Were it *my* dog, and I didn’t want to “bother” with a vet [unthinkable, that] I would start him on 300mg of Doxycyclne, first and watch for 3 days of worsening behavior, followed by better.
Then I would give him a few baby aspirin every day, to see if pain is the issue.
He goes to the vet regularly. I give him Cosequin regularly just as a precaution because he plays hard. If that dog is sick, he's been sick for five years.
I've trained hunting dogs for years, and I've observed the breed. Now that I've actually owned one for seven years now, I can state that the only thing that's wrong with that dog is that he's a pit bull. Every male pit bull I've encountered behaves just like he does. The only difference is he's had the benefit of one-on-one training. But, like all the other male pits I've seen, he's a Jekyll and Hyde who walks on four legs.
To me, they are obviously not a breed you would want for a family dog. Mine even gets along well with and plays with other dogs in the kennel. But, he plays rough, and has to be put out in the kennel with dogs who outweigh him by 20 lbs or more.
You might argue that he should have been neutered, but I can tell you from experience training other dogs that's no guarantee the uncontrollable aggression will stop. A good example is my two best hunting dogs that were German short-hair pointers who were brothers, and were neutered. It didn't matter. Both of those dogs would fight anything in the kennel, and had to be carried in separate dog boxes when you transported them. They would literally fight with anything else you put in the back of the truck.
Our dog likes to be petted, but only if he initiates the contact. The only family member he will not bite hard is my wife. But, even my wife can't touch his legs or feet, or do anything to annoy him or he will instantly grab at her hands. But, because of the strength in his jaws, even that can hurt. He's unbelievably smart, obedient, entertaining, and protective of the family. He's just not a good pet. I've never seen a male pit bull that was, and I always blamed the owners. It's not the owners, it's the breed.
I'm droning on about this now, hoping to convince others who are considering taking on a pit bull for a pet to reconsider, and look at another breed. Pit bulls require a lot of attention and exercise daily. And, by exercise I mean more than just walking. We own a 160 acre ranch, and I can't even imagine seeing our dog trying to survive in a small yard. He would literally explode. And, they can hurt you playing with you, without even trying to hurt you.