I have a Whirlpool estate heavy duty, super capacity washer that has now stopped spinning when it has large loads in it. If you have a small load it will spin entirely out, but large loads will not spin at all... I know this as I have watched with the lid open and the button depressed. Is this a belt problem or something more serious?

I assume that it is stoping because is is getting out of balance. If so, and the washer has any age to it, you probably need a new snubber.