[load balancing] SI Foundry L4-Check / vip flapping

I am triing to understand theses L4-Check logs on a foundry XL (old Version
07.1.08T12 from 2000)
 
The real servers are running but the L4-check seems to often see them down
(never longer than 5 seconds) :
 
Apr 20 18:41:47:N:L4 server 10.0.120.142 idm6c-1 port 80 is down
Apr 20 17:31:50:N:L4 server 10.0.120.142 idm6c-1 port 80 is up
Apr 20 17:31:43:N:L4 server 10.0.120.142 idm6c-1 port 80 is down
Apr 20 17:21:44:N:L4 server 10.0.120.142 idm6c-1 port 80 is up
Apr 20 17:21:44:N:L4 server 10.0.120.142 idm6c-1 port 80 is down
Apr 20 17:01:42:N:L4 server 10.0.120.148 idm6c-3 port 80 is up
Apr 20 17:01:42:N:L4 server 10.0.120.142 idm6c-1 port 80 is up
Apr 20 17:01:38:N:L4 server 10.0.120.142 idm6c-1 port 80 is down
Apr 20 17:01:34:N:L4 server 10.0.120.148 idm6c-3 port 80 is down
Apr 20 16:51:47:N:L4 server 10.0.120.151 idm6b-4 port 80 is up
Apr 20 16:51:46:N:L4 server 10.0.120.146 idm6a-3 port 80 is up
Apr 20 16:51:46:N:L4 server 10.0.120.141 idm6b-1 port 80 is up

 
(there are no L7 check in the configuration, only l4)
 
I am triing to find timeout values or clues about such a behaviour in the
foundry docs, but really can't find any reason why these real servers are
seen as flapping ,The direct connection is working without any troubles.
 
Any help would be appreciated,

--
Youri Pasquier
 
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