Streaming Replication Randomly Locking Up

Hello,

I'm having an issue where streaming replication just randomly stops
working. I haven't been able to find anything in the logs which point to
an issue, but the Postgres process shows a "waiting" status on the slave:

postgres 5639 0.1 24.3 3428264 2970236 ? Ss Aug14 1:54 postgres:
startup process recovering 000000010000053D0000003F waiting
postgres 5642 0.0 21.4 3428356 2613252 ? Ss Aug14 0:30 postgres:
writer process
postgres 5659 0.0 0.0 177524 788 ? Ss Aug14 0:03 postgres:
stats collector process
postgres 7159 1.2 0.1 3451360 18352 ? Ss Aug14 17:31 postgres:
wal receiver process streaming 549/216B3730

The replication works great for days, but randomly seems to lock up and
replication halts. I verified that the two databases were out of sync with
a query on both of them. Has anyone experienced this issue before?

Here are some relevant config settings:

Master:

wal_level = hot_standby
checkpoint_segments = 32
checkpoint_completion_target = 0.9
archive_mode = on
archive_command = 'rsync -a %p foo(at)foo:/var/lib/pgsql/9.1/wals/%f
</dev/null'
max_wal_senders = 2
wal_keep_segments = 32

Slave:

wal_level = hot_standby
checkpoint_segments = 32
#checkpoint_completion_target = 0.5
hot_standby = on
max_standby_archive_delay = -1
max_standby_streaming_delay = -1
#wal_receiver_status_interval = 10s
#hot_standby_feedback = off

Thank you for any help you can provide!

Andrew