How do I stop completely random and uncorrelated barking ‘fits’?

I have a 5 year old intact male pomeranian mix. I got him almost two years ago when he was an abandoned, severely abused rescue who has since transformed into a wonderfully happy, personable, responsive dog – except for one specific behaviour, which I have no idea how to work with: Barking which has no specific pattern except that it only happens at one location.

When I go to visit my parents – whom he ADORES, and who also have a small 9 year old female neutered dog (who is relatively aggressive and resource guards everything in their house from my dog, if that has any relevance) – sometimes, he's very calm and happy. Other times, like today, he kept barking at my parents and their dog. He looks completely surprised and baffled if I try to redirect or use any tricks that have previously worked to stop the barking (certain sounds distract him, petting him when a 'trigger' might come up keeps him mostly silent, at times putting him on my lap stops it) – but it's like the bark is a 'surprise' to him as well. He's totally calm and collected then something snaps and loses it.

Today was especially bad – if their dog entered or left his view, insane barking. If my parents left the room and he heard them walking around outside of his vision, insane barking. When we were going to go for a walk, crazy barking running up and down the house. But there's zero lead up, zero 'explanation' for it, since these aren't consistent every time we visit, and it's not every time he sees them/the dog/they leave the room.

I'm at a loss of how to train this, since it A) is not consistent other than location, B) is ONLY at my parents place, so it can't be regularly worked on outside of being there, and C) is completely random depending on energy levels/his apparent excitement/how many people are in the house/who knows what whether he barks at all. As said, at home, on walks, with other people, he's a happy, calm, silent dog, except for their place – to which my father today angrily told me to "shut that dog up" and commented on how "badly trained" he is. Which broke my heart a little, since obedience/responsiveness/barking is never an issue aside from in their home.

Any tips on how to go about training something like this?