Please help with these error messages

Dear Community need your help once again:

We are running on one installation 2 databases: one for tracking
any activity on a site another for serving out ads based on certain
targeting parameters.

The OS is Linux and the Web Server is Apache

We keep getting around 5-7 requests / sec and activity involves
continuous read and writes into the databases.

Now for some unknown reasons ( I mean unknown to us !!!)
postmaster keeps shutting off..We worked on it for a while and
found one apparent reason to be corrupted data being written...we
rectified that..and now we find that postgres shuts itself off with the
following error messages:

=====

IpcMemoryCreate: shmget failed (Identifier removed) key=5432010,
size=120, permi
ssion=700
IpcMemoryIdGet: shmget failed (Identifier removed) key=5432010,
size=120, permis
sion=0
IpcMemoryAttach: shmat failed (Invalid argument) id=-2
FATAL 1: AttachSLockMemory: could not attach segment

=====

On another occasion we got the following error message:

SISegmentGet: failed: Identifier removed
FATAL 1: CreateSharedInvalidationState: failed segment init
ERROR: cannot open segment 1 of relation sessions_index1
NOTICE: LockReleaseAll: xid table corrupted

FATAL: s_lock(4017aaa4) at bufmgr.c:665, stuck spinlock.
Aborting.

FATAL: s_lock(4000b011) at spin.c:125, stuck spinlock. Aborting.
NOTICE: LockReleaseAll: xid loop detected, giving up
pq_flush: send() failed: Bad file descriptor

=====

Please help.

Also, I wanted to have a quick check on using Postgresql. We are
planning to use it as a large scale adserver where one installation
could need to handle about 75 requests per second on a 24 / 7
basis. This means picking parameters from an HTTP requests
making 5 SELECTS, 25 reads and 5 writes...concurrency issues
already taken care off.

Thanks for taking time off to write to this one.

Kimi