I have a13 year old Plus 80 Model 331AAA which will stop for no reason. Shows error code 14 when it stops. Furnace has been thoroughly checked twice in past two weeks by technicians and they can't find any reason for the unit to stop. If I turn off the power switch on the side of the unit for a few seconds and then back on the unit fires back up and works normally for a while. Unit will also recycle after three hours.Then it stops heating again. No real pattern to this, sometimes in two hours, sometimes in two days. Hot surface ignitor was replaced 1 year ago. Filters are clean and the unit is clean- techs say everything else appears normal. They checked the flame sensor and gas valve- both OK.

Any suggestions? They also visually checked circuit board but found nothing.