Game Shutters After 1 Hour of Play

[Vicetopia]

After I play for about an hour (According to the in-game clock), my game starts to freeze periodically.  Although it's not violent freezing, it's enough freezing for me to notice and it to disturb my gameplay.

 

At first it's only mediocre, but after it does it for 15 minutes it gets unbearable and I'm forced to reboot for another 60 minutes of clean play.

 

Also I've noticed that whenever bullets hit (walls, etc so that the spark & smoke are generated) my game freezes for a split of a second.

 

It doesn't sound that bad, and it's not really, but when you're in the middle of a mission being shot at by 10+ men it really bothers you.

 

It's probably just a cache issue or my memory getting full, that's why rebooting fixes it.  But if there's a permanent fix for this, it would be really helpful.

 

Thanks to anyone who can help.