Memory footprint of Jira increases, then it stops responding

My installation of Jira is regularly requiring restarts. It seems to be due to a memory consumption problem - when started it uses around 60M, but this grows over time (up to 100M within 30 minutes), and once it reaches 160M, the UI ceases to be responsive, and a number of identical errors are logged to "catalina.out". There are also stack traces being logged to the altlassion-jira log - see attached.

I will turn on "debug" logging in log4j and post any relevant extracts when it next fails.