Sounds as though you have a "leaky app"! That is to say, one of the applications you have running in the background is consuming your memory and causing your symptoms. It is gradually increasing the amount of RAM it is hogging instead of refreshing a single segment of memory.
We need to know your operating system. Windows NT and 2000 allow you to see the amount of memory each process is using. With Win 98 it will be a case of using Ctrl/Alt/Del and stopping the applications running one at a time until you find that the problem is stopped. It will probably be a third party program such as a screen saver that is causing the problem.
Get back with a few more details if you can't fix it.