I don't believe that the performance degradation has anything to do with the "free" (which in fact is: unused) memory going down. The reason for it going down is simply because at start time the system has no idea what to put into the cache. Over time it "learns", by loading disk content to memory, which contents are used more often and therefore puts them into cache. This does not slow down operations but in fact speeds them up!

Your problem may come from several reasons: most likely is a "memory sink", which is a pure application problem. The application requests memory dynamically from the system and the system fulfills this request. But the application (being written poorly) doesn't keep track of this memory, so, instead of reusing it or giving it back it requests another chunk of memory from the system - and so on, ad infinitum. Because the systems memory is limited this can't go on forever and over time the systems resources are exhausted. A prominent instance for this happening was/is Firefox. If you run it for several weeks you will notice that it has allocated several GB of memory even if you have only one (empty) window open.

The only rememdy for this (if you can't fire the programmer for sloppy work and then get another one to change the application, which would be the best course of action) is to restart the application (not the system!) at certain intervals. Which these intervals are depends on the application and how rapidly it gobbles up the memory.

I hope this helps.

bakunin