Depending on what make and model it is, it sounds like the drive coupling between the motor and transmission is worn out. Most newer ones are nothing more than a piece of plastic that wears out rather quickly. Engineered obsolescece at its best.
If you go to this site
http://www.repairclinic.com/0001.asp...ne%20whirlpool and click on Repair Guru, you can input your make and model and it will help you diagnose your problem as well as help you find he replacement parts.
If I am correct, it is a $15 part. I just replaced ours recently. Not that difficult for someone tha is inclined. There are instructions available on the repair site to replace it becasue it is a common problem.
HTH
MNS