We went through this issue on and off for years. Tried different settings and we'd be OK for awhile, but the issue always seemed to come back. We have an L260 and a L26500 and are running Onyx.
We came across the "fix" when our HP dealer, who also prints to the trade, told us that they solved the issue by selecting under the HP tab in Onyx and select "HP Everyday Matte Polypropylene". Make sure that you set your printer correctly - should be on "Low temp"