It sounds like the CPU is overheating, and tehrefore throttling itself (slowing down) as to not overheat too much.
Check the CPU fan is clean, maybe clean it all up and reinstall the cpu with fresh thermal paste, you could also monitor CPU usage and temperaturs
based on your explanation, there are many factor contributing to your issue here..
i would suggest you to closely monitor your pc heat and windows background executing programs..
sometimes.. factors can contribute to your problems...
suggest you to have enough ram for your daily pc needs... as software demands on ram has rose rapidly among the past years
hi sages here you need to get the heat down the contributing factor is that when the thermal throttle kicks in is brings down the core clock to help the heat issue i have a c2d at 4.0 ghz and that overheated on the stock fan so what you need to do is what i did google the thermaltake spinq cpu cooler for the lga 775 socket and buy that. it works wonders id know i have it and it has worked at full speed ever since it came out of the box my cpu specs are 2.0ghz fsb and 4.0 ghz core 2 duo processor i achived that on air and you can to is you have that cpu cooler! oh yeah thats right i forgot the 64nm architecture can overheat fast and its slow so if you can upgrade get the c2d