Are you sure you're using CAT-5 cable?
You say the NIC's are forced at 100/full -- how about the switch ports?
Is there adjacent "noise" that might be emitting across the network?
Do you have old vs. current switch HW?  Is it up to date, microcode?
VLAN's -- are you sure it's a point-to-point and not getting re-routed due
to DNS "mistakes" (eg, any potential router involved, multiple DNS entries
for the same hostname, local hosts file on the client, local routing table
on the client)???

Your msg got garbled when stating specifics of your client situation... is
the problem only on one (of many) clients using the same switch?

Finally, what OS platforms (and switch vendor & model) are involved?

These are buggers to solve, unless you can find some consistency -- eg, one
client fails but others run fine (typical, and helps reduce the focus to
identify the delta between good client and failing client -- for Win2K,
we've seen flaky OEM-NIC's cause this kind of problem;  also, one switch
vendor didn't work well with forced 100/full, simply insisted on
auto-negotiate.)