Could be a loose belt that is slipping or bad clutch drive on bottom of the motor. Open the front panel of the washer by inserting a putty knife into the top groove of the front panel in line with the lid opening. Do this on each side to release the two clips. Pull slightly on the front panel on each side as you are inserting the knife. Lean the panel out a little and then lift it off. Observe the pulley/clutch on the bottom of the motor when it goes into spin with heavy load. The pulley, belt, and transmission should all be spinning. If the clip on the bottom of the pulley is spinning but the pulley is not, then the clutch is bad.

Eric

Well, you'll probably need a small puller to get it off the motor shaft. It's not pressed on there but usually they are stuck on there pretty good so a puller is neccessary. Once off, I would take it apart and see if the clutch lining is still good as a new clutch is expensive ($150). If there is still plenty of lining, a good cleaning and lubrication will probably take care of things. Here's a video of how to replace the clutch and at the end it shows how to take the old one apart and clean it. You are supposed to use a new clip whenever you re-install the clutch. Iv'e used the old clips over again without any problems so it's up to you.

Eric