I still think it might be a printer problem. (I had to print out a PDF document recently, which printed perfectly clearly except for a few fields. Cleaning the print heads worked).
Here's another suggestion:
If you've not already got a PDF utility (which lets you convert any printable document to a PDF) download and install this (free) one:
http://www.dopdf.com/
Once you've installed it, view your boarding pass in your web browser. Click on File>Print and change the printer from your usual one to 'doPDF v6'. Click OK to 'print' (which will actually save the document as a PDF) and choose a suitable location (such as your desktop). Then double-click on the icon for the PDF, to view it in Adobe Reader. Then click on the printer icon on the Adobe Reader toolbar to print your boarding pass from there (instead of from your web browser).
Chris