IP Networking

All of sudden in this morning, some computers could not connect to our UNIX server while other still could.
Some computers could ping the server while some could not.
Same on the server side.
It could ping some clients but not some.
All Windows clients could ping each other.

And more, the pining capability became on and off on each clients not simultaneously.

I rebooted the server but same thing.

After a while of such craziness, it seems alright now.

But this is not the first time. I had very similar symptom for a Linux server a while ago.
Back then, pinging a clients from the server somehow kept the connection up.
So, I ran a script that pings several clients repeatedly.
The problem went away by itself after a while as well.

Anybody has seen something like this?
Any idea what has caused such issue?

I would appreciate any help.

What sort of network infrastrucutre is between: hubs, switches, routers, bridges, etc. ? The ping is just an ICMP echo and reply packet pair, and if intermittent, it seems like the network fabric is getting funny, like a bridge crashing and discarding all the ARP or MAC entries for both sides.