Why does ADF scanning dramatically slow down on HP7510?

Hi,

I have a 27" iMAC running Lion and am using the PDFScanner app for capturing scans from my HP Photosmart 7510 multifuction printer. However, I am trying to figure out why scanning slows WAY down after the first hundred or so pages. Here's what happened:

I unpacked my new 7510 and enjoyed a couple hours of scanning and digital filing using the auto document feeder (ADF). The 7510 then started taking about 40 second to scan a single page, no change of settings, would just hang up for 30 seconds with the page half way in the scanner. I thought it was maybe a thermal management issue so I shut it down and let it cool off for an hour. First page out of the gate took that same 40 seconds. I decreased DPI all the way to 100, changed the color to black and white and did everything else I could think of. Scans were fine, just painfully slow.

So I called HP customer support, they suggested I plug the unit straight into a wall because maybe it was starved for power through the UPC. Wierd... but I did it. No help. Here's the kicker: She asked me to take the computer out of the loop entirely and use the ADF to copy a page only. I did, took the same 40ish seconds.

She said there was something probably mechanically wrong with the printer and gave me an RA #. Got it swapped out with a new unit, had another couple of hours of fast, clean scanning (but I went much more slowly this time), and bam, one random page (and every page thereafter) is now back to being 40 or so seconds to scan. It would normally take maybe 5-7 seconds to scan at 300 DPI.

Flatbed scanning does not seem to be effected. So, given that the unit does it when the computer is not being used, this seems like a printer problem... right? Could HP be throttling its users? I sure hope note. Anyone have any ideas?