Depend on what the emphasis is, there are quite a few idioms you can use in this context.
判若兩人 (like a different person)
前後判若兩人 (like a different person from before)
大起大落 (up and down to the extreme/ high and low to the extreme)
峰迴路轉 (surprising, unexpected)
前後反覆 (change completely from before/ turn 180 degree)
一百八十度轉變 (turn 180 ...
CC-CEDICT already has
when words get sour, adding words is useless (idiom)
话不投机 here matches up with “when words get sour.” That's a pretty surefire way to get the editors to approve your definition. (Although the original might need some work).
話不投機 means you don't share similar interesting or thoughts to chat with people or people don't like what you talk about, think it is not interesting.
Regardless the sentences of example you give is true or not, I just consider the semantic since I am not psychological professional.
发现话不投机，好好聊天才是捷径 show that people have the property of 话不投机 so on the latter ...