why slow file transfer speed?

Some times the transfer speed is very slow not even 10mbs and at other times it’s up to speed of 100mbs.This is happening on a small LAN all running win98.I have spoken with others that incounter the same symtoms,but no one seems to understand what could cause it.I have tried rebooting,relogging and even closing all unrelated programs.There is nothing that corrects the problem.Most of the time there is no problem,just once in a while and not always between the same machines.