big pg 6.5 and 7.1 problem in simple application

We have a simple ad tracking application, which has a (mostly) fixed
table size where each row represents a particular ad. We have about 70
rows in the database and use php scripts in apache which connect over
odbc, read a single row, increment a counter, and update that
row. We're performing about 30 updates a second and after a few
minutes the postmaster either hangs or dumps core.

We've tried this scenario on both pg 6.5 and 7.1 on redhat linux, from
redhat's rpms and built from source with the same results. We launch
256 backends with a reasonable shared buffer size. We're using the
unixodbc's odbc driver version 2.0.5. I don't think we're doing row
locks for the query, but that shouldn't crash it - it should just give
us bad data.

The tables, selects, and update calls are all pretty simple, so I'm
baffled by this behavior. Has anyone else seen this problem, or have a
solution?