I'm running close to your specs and I too have a similar problem. I'll be working for about a couple of hours then things will slow down. I haven't been able to pinpoint it. My only guess would be then that there are things running in the background that are taking up resources and taxing the RAM. I usually just quit Maya then re-open it and it's fine again.

Also, check the number of the UNDOs set it Maya. I usually only keep around 20-50. If they are set high then Maya could be the problem. It's working to save all that information so that you can go back and undo it. Try lowering the Undo state to a lower number and see if that helps.