I had the same cutting-on-and-off problem with a Toto/Korky fill valve. Talking with the Korky customer service confirmed that I would need the 528T because of the fill ratio you described. But the replacement cap that Terry mentioned is the same for both valves, and since the toilet was less than a year old, Korky sent one in the mail for free. The new cap solved the problem. Putting your original fill valve back with the cap from the new one might do the job and save buying another fill valve.