Why repeated 5700 error paper jam but no paper jam Pixma MP980?

[marykennedy]

I am trying to print using back feeder. Error 5700 repeatedly shows, tells me to turn get jammed paper out (there isn't any) and to turn printer on and off and if that doesn't work, check manual.

I don't have manual, and manual online didn't seem to have a chapter with troubleshooting, so...

Seems to be a hardware problem; printer was moved recently.

But why does it sometimes print fine and then, the next job, I get the error message though no paper has jammed? 

[mediatrek]

With the unit off, pull the power cord and all other data cables connected (USB, ethernet, etc). Leave unplugged for 10 minutes. This should clear the sensor memory in the printer. While waiting, look in the machine for anything jammed (not just paper, lots of paper dust, etc) from the paper feeding area inside and in the rear by removing the duplex unit from the back.

Plug everything back in after the 10 minutes is up and power up the unit. Hopefully the hard reset of your MP980 does the trick.