GAME keeps slowing down then Freezing :S:S

Hey all....

PLaying diplomacy...2v2v2...with AI....4 hours into the game...the game starts to stutter, slows down and freezes for a few minutes.A few minutes later it all comes back to life, everything starts to move again but at a very slow pace.But again a few minutes after that it freezes.A never ending cycle...no point in playing it since it keeps getting stuck for so long.

I thought the PC might not be be able to cope with it . So i put all video and graphics to low and effects switched of and the same thing happens. 

Any idear what to do ?

Pc specs:

ATI 4870x2

4gb ram

2.6 mhz