whirlpool ultimate care 2 stops, not door switch

I'm not familiar with this machine but if the big knob also turns the washing machine on and off by pulling and pushing it may be that the contacts inside are faulty. They could be burning due to a poor connection and high current passing through them. It wouldn't normally be possible to get to these contacts and it's unlikely to be just dust - more likely burned connection contacts. It would normally need replacing and as you say quite expensive and involved.

However, if it always stops in exactly the same place it may well be something different as poor contacts would be random (unless it always stopped when calling for the heater which suddenly increases power consumption).