@Benmurray1987 sorry you have gone through something similar :(
DH's behaviour has improved, although very slowly. My DM came to stay earlier this month., Before her arrival, I asked that he was nice and calm, and if he couldn't be, to just leave the room/house - it was only for a few days so he couldn't really argue with that. He agreed, but then his irritation started as soon as she arrived. I kept on at him to be kind, and to be fair to him, he did do this. There was one flare up and my mum called him out for being controlling and that seemed to help the situation (!).
I have no idea why he becomes like this when she is around. His behaviour is baffling, and I think it confuses him too (although he'd never admit it).
I guess the one thing I've done, which was suggested by people here, is not to have it swept away. I ask him to explain why he is like it, what the problem is, and rather than get annoyed/argue, is to calmly ask "you seem upset/irritated/stressed/you aren't yourself, what can i do to help?". That helps create a pause and helps him calm down and realise he's being a d*ck.
Overall, it has been a long process, but things are definitely improving.
What has been happening for you? x