CQ5.3 instance can potentially run out of memory due to a memory leak in the SlingServletResolver, maintaining an unbounded internal cache.
This is caused by a Sling issue SLING-1549.
This issue is fixed by CQ5.3 hotfix package #30233. Please raise a DayCare ticket to obtain this package.
After applying the hotfix then you should load test your application and tune the Cache Size parameter here: