postmaster hangs

hi,
i'm using postgres version 7.4.1 and it run very stable until, may be,
4 weeks ago.
now the postmaster hangs sometimes for a while (actually since 2 hours)
and doesn't accept connections but comes back anytime. there is no
possibility to stop the postmaster. even a kill -9 (it's only the
developement system) doesn't work. i have to reboot the system to get
the database running again.
actually ps brings:
postgres 2148 1 0 Mar 11 ? 0:28
/usr/local/pgsql/bin/postmaster -i -d 2

top shows the postmaster process as follows:
? 2148 postgres 154 20 210M 244K run 0:28 0.02 0.02
postmaster

the output of netstat is (grep for 5432):
tcp 0 0 192.168.143.116.5432 192.168.143.116.61145
ESTABLISHED
tcp 0 0 192.168.143.116.61145 192.168.143.116.5432
ESTABLISHED
tcp 0 0 *.5432 *.* LISTEN
tcp 0 0 192.168.143.116.5432 192.168.143.114.2550
ESTABLISHED
45d91540 stream 38 0 0
0 0 0 /tmp/.s.PGSQL.5432
45d916c0 stream 39 0 0
0 0 0 /tmp/.s.PGSQL.5432
40ed2680 stream 38 0 0
43191e80 0 0 /tmp/.s.PGSQL.5432
40ed2b00 stream 38 0 0
4458a400 0 0 /tmp/.s.PGSQL.5432
440c3b80 stream 37 0 0
0 0 0 /tmp/.s.PGSQL.5432
440c3d00 stream 37 0 0
0 0 0 /tmp/.s.PGSQL.5432
449e5100 stream 37 0 0
0 0 0 /tmp/.s.PGSQL.5432
43c65240 stream 37 0 0
4458a280 0 0 /tmp/.s.PGSQL.5432
43a15200 stream 0 0 43b48c40
0 0 0 /tmp/.s.PGSQL.5432
437db7c0 stream 37 0 0
0 0 0 /tmp/.s.PGSQL.5432
437dbc40 stream 37 0 0
0 0 0 /tmp/.s.PGSQL.5432

the server is a pa-risc system with a HP-UX 11.11, 64Bit
does anybody has an idea, what could be the reason for it?
thanks
rolf