Domain logon prblems w/ win98

Hi guys...

I'm having this very strange problem here.... ***sometimes*** some pcs
running win98 aren't able to logon into the domain (win 2000 server),
either windows shows a message that the username or password are wrong
or it says thatIt couuldn't find a domain server... machines running
win2k or xp work perfectly.

There is a firewall running between the networks and the domain
server... at first i thought it could be something related to this,
but then why most of the time everything work fine? I've made some
changes in the firewall rules to clear the way to some pcs, but it
didn't work....

What could be wrong then?

thx a lot in adv. for any help.
Ismael