Large shared_buffers freezing computers

I've had this happen on 2 seperate servers now.

After reading the docs, I bumped up shared_buffers. On one machine with
2G pyhsical ram, I set the param to use 1G of memory ( 131072 value), on
another machine with 800M of RAM, I set the value to about 500M ( 64000
). ipcs shows the correct amounts allocated.

Both servers run fine for a bit, then at some point, the entire box
freezes. Pings work, but nothing else does, so a hard reboot is necessary.

Any ideas. Any limits on what you can set these to. I thought these
values would leave plenty for the other stuff to run on the server.

Here is a top output before freezing:

9:14pm up 38 days, 12:47, 2 users, load average: 4.78, 5.12, 4.91
101 processes: 99 sleeping, 2 running, 0 zombie, 0 stopped
CPU0 states: 24.1% user, 7.2% system, 0.0% nice, 68.1% idle
CPU1 states: 28.0% user, 6.4% system, 0.0% nice, 64.4% idle
Mem: 898892K av, 897300K used, 1592K free, 0K shrd, 0K buff
Swap: 819272K av, 65792K used, 753480K free 805924K cached

PID USER PRI NI SIZE RSS SHARE STAT %CPU %MEM TIME COMMAND
3370 postgres 9 0 382M 382M 381M S 0.1 43.5 10:53 postmaster
32762 postgres 10 0 104M 104M 103M S 15.2 11.9 65:25 postmaster
1226 postgres 9 0 54372 53M 52852 S 0.0 6.0 0:08 postmaster
1334 postgres 9 0 47756 46M 46240 S 0.0 5.3 0:03 postmaster
1181 postgres 9 0 46184 45M 44592 S 0.0 5.1 0:12 postmaster
1227 postgres 9 0 39796 38M 38328 S 0.0 4.4 0:06 postmaster
1228 postgres 9 0 25072 24M 23580 S 0.0 2.7 0:05 postmaster
9082 postgres 10 0 16608 16M 15180 D 5.0 1.8 0:00 postmaster
9084 postgres 10 0 14700 14M 13316 S 4.6 1.6 0:00 postmaster
3244 postgres 9 0 13376 13M 12052 S 0.0 1.4 0:00 postmaster
32668 postgres 9 0 11488 11M 10224 S 0.0 1.2 0:02 postmaster
32669 postgres 9 0 11136 10M 9888 S 0.0 1.2 0:55 postmaster
9085 postgres 15 0 10820 10M 9520 S 2.5 1.1 0:00 postmaster
9087 postgres 18 0 10796 10M 9496 R 2.9 1.1 0:00 postmaster
9086 postgres 16 0 10696 10M 9400 S 2.3 1.1 0:00 postmaster

Thanks,
Michael