I'd need more info. Your exact cpu, or psu, ram, and hard drive.
-If you have a low psu it could be bottlenecking the hard ware
-If its a bad cpu it could be bottle necking the gfx, im guessing its the iix4 from some company, because its a quad processor wouldn't exactle mean it's good with gaming, some quads are good in utilities because of hyperthreading, but has low cache, so it's bad for gaming.
-If its your hard drive then the low rpm from the hard drive could make loading times of the maps slower, decreasing fps. if its your ram, then get more ram.