Connections with virtual server lost?

You could start checking the statistics of your tcp profile (virtual server -> statistics -> select tcp profile at the bottom). Maybe there you can find some counters pointing you into the right direction. Also, the /var/log/ltm logfile might contain useful information. If that doesn't give you any insight, try capturing those faulty connection attempts with tcpdump. If you cannot see the erroneous connection attempts there at all, some device before the BigIP might be causing problems (router, firewall...).

HTH

Martin