Lots of things can do that tip up the printer to see if you can some
thing jammed in the Feeder, that if its not a hard jam or see if the
Paper feed roller is working.
I have a similar problem with a R310 does it now and again, but not just
the odd page, have to strip it down to see what is wrong.
If you are technical at all this might help as they do list fault
problems and what to look for
Take a look at the paper feed path to make sure nothing has fallen in the
way. You might even lift the printer and tilt it backwards to see if
something hasn't become lodged in the path.