out of memory error

I am currently running a load test of our systems using soapui runners involving fixed loads. no more than 16 threads. I am encountering the 'out of memory error' box after about 20 mins. I have read through the forums and tried a few ideas, like increasing the concurrent samples on the soap ui runners and workspace max threads but to no avail. Can you offer some advice or let me know if any other information may help.
Thanks
Luke