Fault with CD-309 / 9 cd tower duplicator

When using a tried and tested master cd the duplicator will begin to burn to tried and tested media and continue to do so for well over an hour before we have to switch the power off because no other buttons will work. The blank cd-rs remain blank. Having switched the duplicator back on the burn takes place perfectly with exactly the same discs involved. This will occur with a frequency of 1 in 10 burns. We have tried to upgrade the firmware to v3.18 to erradicate the problem in the past, but clearly this has not worked.