Calibration problem

My HP 2600n prints great at the top of the page. But from half way down the page on, the print quality deteriorates. The colors, images and even text become skewed more and more in the lower portion of the page. I’ve calibrated repeatedly, which helps but doesn’t completely fix the problem. And it only lasts for a short time and the problem returns. Any suggestions for me? I bought the printer a short time ago – it was refurbished.