Printer loses page centering when printing photos

I have a photosmart 8150 that recently started drifting off to the edge of the page while printing images. I thought the paper might be slipping, but after cleaning the problem remained. I checked several times during the print and the page is exactly where it should be but the printout gradually runs off the edge of the page as the printing progresses. Obviously, the print head has lost its ability to maintain centering on the page. Not sure if this id done by a sensor that might need cleaning or if it is done through a step motor function (slipping drive belt or gears?) . Anybody have any ideas?

Thanks!