You mean the parts in the tank have all been replaced?
Go to the hardware store and buy a pressure gauge that you can screw on the hose bibb at the front of your house. The readings should go to at least 100psi.
Do your own test, screw it on the hose bibb and turn it on. What does it read? If over 70#, the pressure to the house is to much and probably causing your fill valve to slam shut before it goes off. Installing a pressure reducer on the cold water service to the house and setting it at 50#psi should take care of the problem.
If the pressure gauge already reads 50# or lower when you do your test, let me know and I will scratch my head a little more for you.
The Local Plumber
Tustin, California http://www.TheLocalPlumber.com