Top layer issue

Hi,

I am having issues with the top layer finish on my prints and would appreciate some advice. I use 2x X1C.

Currently have 5 top layers set and the first 4 layers print perfectly, but for some reason, the very top 5th layer creates this weird wavy pattern. Not sure where to look to try and fix this. I have printed lots of these designs without any issue in the past but now it seems to be a common occurrence.

Cheers, Rory