PC3348 STP/flapping problem?

I have a 3348 at a customers site (400 km away unfortunately) that misbehaves.
It is setup as a single VLAN as per default. I have access to the switch via
VPN and https/ssh.
The customer experiences occasional (5 times during working hours today)
outages in network communication. They are using thin clients, so this is
especially annoying. These outages last upward from 20 seconds.
I have tried telephone/email support here in Germany to no avail.
Disabling multicast, storm control did not better the issue.
Browsing your forum, i noticed

http://delltalk.us.dell.com/supportforums/board/message?board.id=pc_managed&message.id=2872

where you mention that recurring DOWN/UP sequences could block network traffic.

I have only a single switch, there should be no loops so i have tried your
suggestion (for a test)
(config)#no span
and see lots of
Nov 19 20:59:31 192.168.223.253 %LINK-W-Down: 1/e11^M
Nov 19 20:59:33 192.168.223.253 %LINK-I-Up: 1/e11^M
Nov 19 21:02:18 192.168.223.253 %LINK-W-Down: 1/e9^M
Nov 19 21:02:19 192.168.223.253 %LINK-I-Up: 1/e9^M
Nov 19 21:02:33 192.168.223.253 %LINK-W-Down: 1/e11^M
Nov 19 21:02:34 192.168.223.253 %LINK-I-Up: 1/e11^M
Nov 19 21:02:54 192.168.223.253 %LINK-W-Down: 1/e9^M
Nov 19 21:02:56 192.168.223.253 %LINK-I-Up: 1/e9^M
Nov 19 21:03:45 192.168.223.253 %LINK-W-Down: 1/e11^M
Nov 19 21:03:47 192.168.223.253 %LINK-I-Up: 1/e11^M
Nov 19 21:07:08 192.168.223.253 %LINK-W-Down: 1/e9^M
Nov 19 21:07:10 192.168.223.253 %LINK-I-Up: 1/e9^M
Nov 19 21:07:44 192.168.223.253 %LINK-W-Down: 1/e9^M
Nov 19 21:07:46 192.168.223.253 %LINK-I-Up: 1/e9^M
Nov 19 21:07:59 192.168.223.253 %LINK-W-Down: 1/e11^M
Nov 19 21:08:01 192.168.223.253 %LINK-I-Up: 1/e11^M

in our log. Is this what you call flapping?
These are Dell server RAQ ports. Could these be cause of outages
as described, and what would be the proper cure?

Before the log looked more like:
Nov 19 08:29:31 192.168.223.253 %LINK-W-Down: 1/e44^M
Nov 19 08:29:32 192.168.223.253 %LINK-I-Up: 1/e44^M
Nov 19 08:30:03 192.168.223.253 %STP-W-PORTSTATUS: 1/e44: STP status Forwarding^M
Nov 19 08:43:52 192.168.223.253 %LINK-W-Down: 1/e32^M
Nov 19 08:43:54 192.168.223.253 %LINK-I-Up: 1/e32^M
Nov 19 08:44:25 192.168.223.253 %STP-W-PORTSTATUS: 1/e32: STP status Forwarding^

Thanks for any info

Wolf Schmidt