Parallels Desktop slows down after an hour of use?

Hello all. I use Parallels Desktop to play PC games that cannot be played on the MAC.

When I first start up PD, it's a bit slow when starting up (then again, what cpu or program isn't? Am I right?) which is fine, it then goes to normal speed after a minute or two. PD runs fine and everything, however, after an hour of usage, PD begins to slow down. The only way to fix it is to restart PD and not the MAC. After restarting, it runs fine again for another hour of usage.

Do any of you maybe have some tips to simply extend the time of usage of PD?

* I have a MAC 10.5.8
* I close everything else on the MAC when running PD.
* I have 170 GB free on my MAC


Thank you to anyone that helps.