There are two possible solutions worth trying. First, try resetting the parameter RAM (PRAM). PRAM could be causing some of the numbers to be misrepresented. (Search for "PRAM" in this database for how to reset PRAM. The procedure is different for System 7.x than for System 6.x.)
If the problem persists, throw away the System Folder in use and reinstall from known-good, locked, original System diskettes.
Finally, if none of these works, have the logic board checked. It may need to be replaced.