Thanks for all the replies folks. I certainly didn't expect Dave's post, very cool pic - hope you got the problem sorted.

This colume spike issue is kinda weird to diagnose because I think I'm seeing conflicting symptoms - it certainly does appear to be worse when it's damp, but it also only appears to be happening with Newstalk. Maybe it's just that I was listening to Newstalk when it was raining, I need to test other stations next time it's happening again (but it's never been as severe as in that recording). Another thing to mention on it being station specific is that I'm usually in Mayo, but I also see this issue when back in Dublin.

I was watching and noticed that when the volume spike hits during bluetooth play, the radio display text changes from "CD 5-01" (or whatever track number it happens to be on) to "106.8 FM". It just changes for that split second during the spike, and then changes back to CD mode (which the Dension unit uses for bluetooth).

Are these conflicting symptoms of potentiator vs some kind of radio frequency or RDS?

I'm going to try disabling RDS and see if that helps.