Strange banding issue

[Josh Ledger]

Hey guys! I have been engraving mirrors and have run into an issue as of late. The mirrors that I'm engraving are 12" x 12" and the artwork is about 10.5" x 10.5" vector. I sometimes get banding on the "larger" files (they take longer to load and save than other files). The banding is always in the same areas of the artwork, no matter where the mirror is placed in the machine so this leads me to believe it is a file issue. What can I do to fix this issue? I have a "50 watt" 500 x 300 mm china laser from ebay running RDWorks V8 and using Adobe Illustrator CS 5.1 for designing. I have tried different power settings to see if that's the issue but the banding is still there. Here is a picture of it: 335488
Thanks in advance for any help or suggestions!