Dishwasher jumps timer Bosch Exxcel?

My Bosch Exxcel Dishwasher is 4 years old and doesn't finish the cycle. The timer jumps from say 90 mins left down to 50, then goes normally to 6 mins, then up again to 35. It keeps on doing this, not arriving at 0 at all. It never ends up drying, and what has been washed is not all too clean either.

Ruth
October 2009