Server drops all users for a few minutes then goes back to normal

Server becomes unreachable for a few minutes.

Netware 4.11 small business with 2 Novell-mirrored scsi drives.
clients: Win98 (about 25 of them).

Tuesday July 24 around 2pm, all users lost their connection to the server.
After a few minutes, they rebooted their computers and everything went
back to normal. I checked the server's log and there was no error reported
there. I searched the server's hard disk for files modified in the last 10
minutes looking for maybe another log, but there was nothing else.

Thursday July 26 around 2:18pm. Same problem, same solution as Tuesday. I
was able to go to the site and there were 5 messages on the console, all
stamped with the same time (7/26/7 2:18:42pm). Message:

Server-4.11-911
A scheduled "Work To Do" took over one minute to run.

Again, no errors logged at all anywhere.

I googled the problem and found roughly 3 answers:

1. Install patches.
2. Replace corrupt driver(s).
3. Check hardware (hard disks, cables, etc.).

Regarding #1, this server has been running without any changes in software
or hardware for several years now and this is the first time this happens,
so I have doubts that patches would resolve the problem. Of course,
correct me if I'm wrong.

Regarding #2, how do I find which drivers are corrupt? Step by step
instructions or a link to them would be extra nice.

Regarding #3, My first guess here would be a hard disk cable. Am I being
realistic?

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Thank you!!!!!

Alex