Are you definitely not using MDEX 6.1.3 (you entered this originally then edited it)? I could see this happening on 6.1.3 if you have wildcard search enabled - there is a bug in that version that causes a memory leak (when a customer makes a wildcard-only search that gets past any web application checks for minimum characters, e.g. ***). No idea what could cause this in 6.2.1. Is this the same issue as you previously reported, and did you follow any of the steps outlined by the respondents on that thread?