First certainly is a switch problem which will be located within the switchbox assembly. A replacement switch is only about £4.00, so if you can change it yourself then it would be worth investigating.
Yours could be more serious- probably not the switch as the either work or they don't, so not likely to start to cut out. Could very well be the motor- these have a habit of bearing failing which causes them to tighten up and gradually slow the motor to a stop. Generally you will get some smoke and/or humming coming from it. Another fault could simply be a work hardened break in the mains cable (the one from the plug into the switchbox- to test this, plug it in, press the start button and pull the switch lever- whilst holding the lever in the on position, with you free hand roughly whip the mains cable around- especially near to the switchbox- if there is an internal break the mower should switch on and off.
pleasant
September 2008