Doesn't sound like the microswitch to me - easy to check though with an insulated screwdriver and a meter.
Your problem sounds similar to my Essentials - the shower makes a horrid bass sound and then cuts out (usually mid lather!).
2 years ago it was the carbon brushes, which I replaced for 20 GBP. This time the motor is really hammered (I think it was installed off-centre and is worn heavily in one side) and I think I need a new motor unit. The motor unit is part 419-50 and about 70 quid.
If anyone knows where to fin a cheaper one, please let me know.
Pete, November 2008