Although other programs are printing correctly to the Laserjet 2420, your tests indicate to me the problem is with the printer driver.
If I remember correctly, the Laserjet 2420 can do PCL and Postscript so try experiementing with PCL 5, PCL 6 and PS. Also, if the printer is on a print server, try directly connecting it to the PC. I would also get the latest driver from HP.
I've never had to do it, but they do make ROM updates for the printer.
Be sure to post the solution when you find it.
Mike Honeycutt