If it is indeed 5TPI screws then you should set the motor steps per revolution to 314.9606:

You say 320 step/mm and 5mm pitch, so 5*320=1600 step/rev. That implies your motors are set to 1/8 microstep... 5TPI is 25.4/5=5.08mm. So 1600/5.08=314.96... step/mm.

Try that and measure it over a greater distance. What are you using to measure the travel?