Mfc-6910dw Paper Feed/alignment Issue

MFC-6910DW printer has issues when printing from the manual feed tray at the back.  When the paper is printed, the horizontal alignment is off.  For example, if a horizontal bar goes across the page, then that bar will be higher on one side and lower on the other side.  This issue does not happen when paper is fed from any other tray.  My guess is that either the paper is being pulled through too fast on one side, or maybe there it wiggles as it's pulled in?  The guides are snug but not tight, the media is acceptable.  Sometimes the media wont be pulled through at ALL but that is more frequent with thick paper that is like card stock (still acceptable media for this model).  I got my hands on the service manual and I've tried looking at all the rollers, but I know some of the rollers won't even come into play, such as the lower tray pickup rollers and the Switchback roller.  I'm not sure what to look for in terms of damage to a roller.  The pads do not look worn and they are not dirty.  The shaft for the rollers does not seem to be loose.    Any suggestions?  Any way to test if it is a engine problem?