Bikermouse has a good suggestion....  there might be something wrong with the power supply..   These things can become faulty and can start emitting electrical pulses that can cause problems in the timing circuits...  

One cheap fix is to find a dis guarded comparable computer and swapping power supplies..   make sure the power supply has the same or better power...  In NYC you can find them on the streets in the side walks along with the garbage...  Maybe your town has them too...

This does not sound like a hard drive problem... 

just things to try... if you already tried these skip the suggestion.. 

1)  See if the computer is clean inside...  vacuum carefully...  use clean brush...
2)  take the graphics card out and see if the computer runs more smoothly....

3. check the power cables... for splits touching the metal of other parts like the casings..  replace if in dough...
4. check the hard drive cables...  Make sure they are seated correctly.. clean them..  
   replace if needed...
5. check the hard drive sockets... 
6. check sound card pins, cables and sockets..  replace if need be
7. check mother boards pins sockets.. and power cables...  replace when applicable

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