parallel blank lines in my aerial photo print

Check out the scan of the image I am referring to here:

http://www.wright.edu/admin/construction/images/dave/lines.jpg

I am using a HP DesignJet 500ps

I have aligned the print heads and run the calibration on the plotter. But I still seem to get prints with these lines in it. But the thing is it is somewhat sporadic, I can come in one morning and they print fine.

Anyone else run into something like this before?

Thanks,

Dave