It's not the liner it's the machine the wire when looking in at it is stopping and starting and then stopping completely I have called rtec will cost me 45pound postage each way the worst case few hundred to fix could end up the price of a new one
First id check if the motor was stalling or not, i'e does it turn with no wire in it or does it stop? if that works file then its a wire/torch issue. if the motor stops with no wire then its a motor or power issue.
then the next step is based on the result of that test. e.g. like @bwservices said test without the torch, but with wire in the drive wheels. if the motor is stalling measure the voltage at the motor is this steady? is the voltage when the motor is stopped or not?.. Following this methodology will lead you to the root cause of the fault
Rtech should take the time and talk you through fault finding to resolve the issue not just fob you off and expect you to send the whole welder. That is the service I got from tecarc and meant just sending the PCB back rather that the whole welder.