My suspicion is either a memory leak somewhere in the software stack or heat. Both of those just because I haven't any better ideas.
To determine if you have a memory leak:
Run top
If you are using increasing amounts of swap space you probably have a memory leak.
run top like this (batch mode, 1 iteration) to get a snap shot of process memory usage. Save the output and run it at different times to get an idea of what maybe eating memory.
