Sounds to me like sticky fill solenoid. Common thing. Cheap fix. Easy replacement.

I would have worried about the control-board or relay but your description of intermittent problem combined with normal operation most of the time leads to focus on the more-common problem. When control-boards or relays fail they generally stay that way. Solenoids love to drive everybody nuts for a year first. I'll bet that's what you've got.

Thanks for the reply! I've searched online but can't find "fill solenoid" for my Bosch. Would it be called something else? I've found "Water Inlet Valve", is that the same thing?

Yes

There is also a fill switch which is supposed to shut the water off once it reaches the level you select. I would suspect that before a stuck solenoid.

Hi don_1_2006. I've searched for a fill switch and I might try that as my first solution. But I'm having trouble finding the "fill switch". Is this the right part?
http://www.repairclinic.com/PartDetail/Sensor/491681/1107396

I'm still searching for a "fill valve" and can't find anything and was wondering if anyone can shed some light. The exact model of my washer is Bosch Wfmc8440UC . Thanks.