What's causing my washer to stop running during the wash cycle?
The machine is not emptying properly for some reason.  All washing machines are designed never to spin whilst full of water.  The default is either to pause indefinitely or reset the program.  It's odd that the machine is fine when empty though.

I would still do the following before calling out a repair company.


Check that it isn't blocked or partially blocked.  If the machine has a service panel at the base then it might have a round filter dial that when turned comes out of the machine (plus old baby socks, coins etc).  Make sure to grab loads of old towels and put them around the base of the machine before doing this.  The machine might have several litres of water or more still in it.  If you can see water inside the machine then you will need a lot of towels when unblocking the filter... you have been warned ;)  Get a well insured friend to do it otherwise :D
Make sure that the grey waste hose at the back of the machine is not trapped.
Make sure that the grey waste hose is not higher than the machine or pushed too far into the drain pipe.  This can cause water to back up into the machine and complicate matters.
The machine might have a coin lodged in the pump or something similar.  Listen for any rattles when it is emptying.  You should at least hear a churning sound coming from the pump.
Give the door a push in case the lock is faulty and the clothing is pushing outward causing the lock to break a circuit.
Check that both Hot and Cold water are actually going into the machine.  If you have turned the Hot off at the wall and the machine has an economy mode then it is waiting for hot water.