PL/PGSQL functions suddenly not working

Hi,
I have a couple of functions in my database (v7.3.4) and noticed that 
after a certain period of time, the functions fail to work This happened 
now serveal times and the function fail at different points. The did go 
away when I restarted the postmaster.  At one time the function always 
failed when another heavy job was running on the system.

Is this a bug? Does anybody have the same experience ?

Thanks
Alex