Why does access point need rebooting?

Hi,
Why does rebooting the wireless access point speed the network system up?
Have a wireless network with about 15 users. On a regular basis (every day or two) it slows way down so users complain. Rebooting the wireless access point temporarily solves the problem (network speeds up again). We are trying to troubleshoot/solve this problem.
I am guessing that the access point has a memory area that gets clogged up over time. If that is the case, is there some way to prevent this? is there some other explanation?
We are trying to troubleshoot our problem, but we need some guidance as to how to approach it.
Thanks,
Mike