Queries start failing after loading large results via cursors. Server restart helps but after a number of queries error comes up again.

Collection contains 15Mio documents including text fields. Queries fail from mongo shell and using the java driver:
> db.WPI2.find().count()
Tue Aug 16 17:36:35 DBClientCursor::init call() failed
Tue Aug 16 17:36:35 query failed : test.$cmd { count: "WPI2", query: {}, fields: {} } to: 127.0.0.1
Tue Aug 16 17:36:35 Error: error doing query: failed shell/collection.js:150
Tue Aug 16 17:36:35 trying reconnect to 127.0.0.1
Tue Aug 16 17:36:35 reconnect 127.0.0.1 ok

Mongod allocatates about 10GB of memory (Resident set but shared). Server is not swapping, though.