An audio record of it can be listened to here:
https://d.pr/a/TklCXj
(DropLr if that is not cool I am happy to post it somewhere better)
After about 5 - 10 minutes it would start this odd phase warping.
I am sure that is the wrong term. I have not come across this before
I have changed out (all) audio cables, different speakers, various sources, different channels, different outputs. it keeps happening.
I hooked up a different mixer, same cables, sources, speakers, had it for two weeks now and it has no issues. (Its a loaner so I cant keep it)
I took it to the shop and they said they had no issues.
I take it back and I still have the issue.
I went to a PA place and harassed the guys to listen just to the recording and they had no idea either.
it is still under warranty but until I can get the shop to recreate it they wont take it.
Could it be an electrical wiring issue? It runs off an 240VAC adapter to 19V DC.
The shop will sometimes lie unless you can bring in a recording with two identical signal chains and only your mixer different. But it's also possible that you have a ground loop if your equipment is not all properly earthed. You didn't say if the different mixer which sounds OK is the same make and model as the problem unit. If it is then good, you have a simple demonstration of what's wrong. If it's two different kinds of mixer then it could be a design issue.