Paper jams only while duplexing

Single sided printing works great. When ever duplexer is used, first side prints fine but jams when it is sucked back into print second side. Have replaced duplexer unit, dc controller, drivers, have had a tech out for hours working on it to no avail. HELP!! You can actually watch the papers go through the duplex part of the machine. Suddenly one will bend at a corner and cause a jam. There is also an odor like the machine is hot. This machine is less than 2 years old.