IM Store thread hanging issue

On our Exchange 5.5 NT 4.0 system for about 50 users, my IMStore process has been running once a week without problem until several weeks ago. Now it is hanging up and causing a thread to go to 100% CPU utilization. I have checked and followed several of the MS Tech articles but it is still doing so. I have checked the users PST files for problems and have only found one so far. This particular one is the connector to our faxing software. It was repaired through the Inbox repair tool. Currently I have the IMStore process turned off and I am looking for suggestions on what to do next. Some of the suggested repairs have lots of warnings and I am not sure if that is the correct approach.