developer 7.1.1 disconnects continuously.

developer ver. 7.1.1
IS ver.
OS : AIX ver. 5.3

I think developer loses its session.
developer disconnects from IS even without IS restart.
I attached trace log of when developer disconnects.

When developer loses its session alert message window
pops up - ‘Session lost / Session restored’.
and it occurs normally once in an hour, but it doesn’t repeat regularly.
sometimes, it disconnects more than 4 times per hour.

I think it’s not a network problem. I tested in local IS and developer.
then, I got same result - developer disconnects.

Have you ever seen this before? any ideas?
Thx in advance.