C++ Space leak, Multi-threaded application.

I have a multithreaded (Max threads = 50 (limit) application which takes job requests process it and returns results. When the application starts total memory (RSS) is 15 MB. For each job request, memory usage starts growing. If I submit job every second and ran application for 2 days, total memory usage raises to 300 MB.

I did not find any memory leaks by using valgrind. Also did static analyis "CppCheck". So I wrote my own memory leak detector by overriding new/delete and tracking all the heap allocation.

Application heavily uses: std::stringstream class. I do know understand why memory consumption is increasing.