Crazy number of "Resets sent" TCP segments

Hello,
I have Netware 5.0 server with B57 Broadcom NetXtreme NIC and TCP console
show me one third of sent segments as flagged with RST (TCP RESET).

I have a suspicion, that this situation slows down backup operations with
Arcserver tape. When I disconnect ethernet cable, the backup runs much more
faster.

I'd like to sniff some of these RST packets for debug, but I haven't found
sniffer for 5.0 (is PKTSCAN.NLM confirmed for 5.0? Documentation show 5.1
only....)

This server acts as mailserver, but now, Groupwise is down, no other
services except Arcserver backup are running.

Help, how to get rid of RST packets is appreciated, thank you.

Jakub Talaš