The breaker that trips - if it's the large two-pole breaker that turns off all power and lighting - is the RCD (or similar) intended as a safety device to prevent electrocution.
The machine's heater is unaware of the weight of the load put inside - the machine's motor will feel the extra weight.
It follows then that you may have a motor related fault - many people change the motor's carbon brushes when things begin to go wrong - sometimes it's a fix. Brushes are relatively cheap and relatively easy to replace.
Here's a video of brush-changing on a bench - probably because the camera would not fit inside the machine.
http://www.youtube.com/watch?v=0GmtRBiXLFc
Good luck...
Peccavi
March 2010