Hello again...

I have a mysterious issue that I cannot figure out. Any advice we be greatly appreciated.

I am "engraving" the paint off of anodized aluminum alloy panels to create patterns and images.
I am running into a problem where the output is uneven especially if the engraving takes more than 5-10 minutes to complete.
I get random striping off and on throughout the piece (please see photo).
I found that if the banding starts and I pause the engraving and then re-start it the burn is clean again for a few minutes.
When it is banding it looks as though the laser is less bright on the surface.
This suggests to me a possible power issue, but what kind I don't know.

Attachment 330814

Has anyone seen anything like this?
The example was engraved at a speed of 100mm/s at 50% on a 40w chinese co2 laser.

Thanks!