probably 1 of 2 things:
1) the flapper is not closing
2) the fill valve is not shutting off
When the problem is happening, take the lid off the tank and look inside. When the tank is as full as it should be and still running, lift up on the float. Did it stop running? If so, you'll probably need to replace the fill valve. (The float "shuts off" the fill valve.)
If the tank never gets filled to the proper level, replace the flapper.

[This message has been edited by rpxlpx (edited May 12, 2003).]