I have this same reoccurring nightmare and I also grew up in the UK.
The line would be smooth, but the violently contort into anxiety inducing sharp triangles.
This is the first time I have heard someone echo the same dream.
In a way perhaps it's kind of an abstract "powerlessness in the face of chaos" nightmare.
Not sure why exactly it went away. But I do remember being conscious of it in my 20s and trying to understand it. I think I tried to understand it consciously and accept some of the powerlessness perhaps. Or maybe later on, it was meaning general confusion and anxiety which was suppressed.
At any rate the nightmare was very scary and awful, as if something evil and wrong was there.
