Next time it starts making the noise, lift up the lid and press the lid switch with a pen or pencil. Watch the agitator and see if its stopping or jerking instead of operating smoothly.

I've seen a few machines over the years that would work OK on small loads, but have a fit on larger loads. Usually they make a rather loud rasping noise or maybe you might call it a ripping/grinding noise when they are not agitating.

The noise and failure to agitate are all caused by the gearcase. The gears inside are slipping or have their teeth worn down until they are grinding against one another.