bash: command not found after piping

Hi,

I'm trying something quite commonly found in an Unix environment, namely piping the output of one command into the input of a subsequent one, eg.

bash$ ps ax | grep -i bash

But unfortunately I'm receiving--and now comes the most curious part--most of the times (not always!!) an error message

bash: grep: command not found

When calling grep directly

bash$ grep

the resulting output is

Usage: grep [OPTION]... PATTERN [FILE]...

Try `grep --help' for more information.

"which"-ing the commands

bash$ which bash ps grep

results in

/bin/bash

/bin/ps

/usr/bin/grep

Cheers,

MMHein