FATAL 1: Memory exhausted in AllocSetAlloc()

Hi there!

The last db-crash (kernel panic) caused a little error on my db-files i
think. After the crash i had to recreate some indexes but no data were lost.
Everything is running quite ok now, except for one BIG error.

The postgres process crashes every now and then without giving any
information on any reasons. :( AND I can't use pg_dumpall, vacuumdb -z. All
the time I run a program that is supposed to dump my database or analyze it.
I get the following error from the program:

FATAL 1:  Memory exhausted in AllocSetAlloc()
pqReadData() -- backend closed the channel unexpectedly.
        This probably means the backend terminated abnormally
        before or while processing the request.
connection to server was lost
vacuumdb: vacuum failed
pg_dumpall: pg_dumpall failed

I'm running a postgres-7.0.3 on a p3-800 with 768mb ram. postgres is the
only thing running on that machine. The Database Directory is 500mb big. I
have several tables (some arround 5k entries and about 5 tables are >100k
entries). The DB is heavy loaded (linux load of 6-20 at main time) and I
plan to move it to a bigger machine. The only problem is that i can't dump
my db. :(

Somebody got any advices how I can dump or vacuum my db again?

thanks in advance,
thomas