I hope that you are using HP genuine toner cartridge and have configured the tray with correct type (Letterhead) and size of paper. Use the same paper type and source tray selection from printer driver properties while printing your job.
Please try to print letterheads from another tray than previously used and verify results.
Try to print with low engine speed (quiet mode = ON)
If you could attach a scanned file of print sample with your post, it will help us for further troubleshooting.