Out of memory errors on Confluence

Confluence site stopped yesterday afternoon with out of memory errors.

Restarted ok

We have setup Confluence as a windows service:

"%CATALINA_HOME%\bin\tomcat.exe" -install Confluence
"%JAVA_HOME%\jre\bin\server\jvm.dll"
-Djava.class.path=%CATALINA_HOME%\bin\bootstrap.jar;%JAVA_HOME%\lib\tools.jar
-Dcatalina.home="%CATALINA_HOME%" -Xms256m -Xmx512m
-start org.apache.catalina.startup.BootstrapService
-params start -stop org.apache.catalina.startup.BootstrapService -params stop -out
"%CATALINA_HOME%\logs\stdout.log" -err "%CATALINA_HOME%\logs\stderr.log"

stderr.log:

java.lang.OutOfMemoryErrorjava.lang.OutOfMemoryError
java.lang.OutOfMemoryError
java.lang.OutOfMemoryError

stdout.log
starting outputting lines like this.....

2005-08-21 16:46:02,348 WARN [sf.hibernate.cache.ReadWriteCache] An item was expired by the cache while it was locked (increase your cache timeout): 267
2005-08-21 16:46:02,348 WARN [sf.hibernate.cache.ReadWriteCache] An item was expired by the cache while it was locked (increase your cache timeout): 267
2005-08-21 16:46:02,348 WARN [sf.hibernate.cache.ReadWriteCache] An item was expired by the cache while it was locked (increase your cache timeout): 267
2005-08-21 16:46:02,348 WARN [sf.hibernate.cache.ReadWriteCache] An item was expired by the cache while it was locked (increase your cache timeout): 267
2005-08-21 16:46:02,348 WARN [sf.hibernate.cache.ReadWriteCache] An item was expired by the cache while it was locked (increase your cache timeout): 267
2005-08-21 16:46:02,348 WARN [sf.hibernate.cache.ReadWriteCache] An item was expired by the cache while it was locked (increase your cache timeout): 267
2005-08-21 16:46:02,348 WARN [sf.hibernate.cache.ReadWriteCache] An item was expired by the cache while it was locked (increase your cache timeout): 268