With the information provided I think the best you're going to get are guesses. Might be worth taking a look at a good logging utility like log4c. You have the ability to pass all sorts of objects like arrays and vectors to it and it will display the contents of them giving you an easy overview of if their contents is growing. Adding some lines of debug code at various points in your code is another obvious solution.

You mention CPU usage increasing. Does memory usage also increase too? If so think about implimenting a Memory Printer class that peridocially prints the used/available memory.

Another thought, and one I've come across quite a bit within my own company, is your program talking to some sort of database? Is it performing read and writes to a specific table that is getting bigger and bigger. If you have a thread that is querying a table at a given interval then obviously the more rows in the table you have the poorer the performance of your application will become, well without proper indexing anyway.