Balance and noise problems in washer

Washer is noisy and out of balance on large or heavy loads and I have to readjust the clothes to keep it spinning. It often does not complete the wash cycle with anything but a small load. It violently shakes and there is a loud knocking sound on the spin cycle. It does make a moderate screeching sound when wash cycle first begins and ends. The belt was replaced in November 2011. Any help will be appreciated:)