Moxi proxy downstream timeout SERVER_ERROR

I have a live cluster with two Couchbase nodes (version 1.8.0) and about 5 application servers each running PHP with moxi clients on them. Once in a while, Moxi will return a SERVER_ERROR when attempting to access data. This happens about once every few minutes on average. The cluster processes about 500 operations per second.

After inspecting the moxi logs (with -vvv enabled), I notice the following at around the time I get a SERVER_ERROR:

2013-07-16 03:07:22: (cproxy.c.2680) downstream_timeout
2013-07-16 03:07:22: (cproxy.c.1911) 56: could not forward upstream to downstream
2013-07-16 03:07:22: (cproxy.c.2004) 56: upstream_error: SERVER_ERROR proxy downstream timeout^M

I tried increasing the downstream timeout in the moxi configs from 5000 to 25000, but that doesn’t help at all. The errors still happen just as frequently.

Can someone suggest any ideas for me to discover the cause of the problem? Or if there’s some likely culprit?