I just went through this myself. Inside the cap there is a rubber gasket (for lack of a better word) that has developed holes and is cause this spurting. Repair is simple. Turn the water off, flush the toilet, remove the rubber tube, remove the blue valve cover, and take the cap off. Get a new cap and reassemble. I had a hard time finding the cap cause Smock Man at HD said they didn't make just the caps. I knew that wasn't the case, but I took his word and tried elsewhere to no avail. I finally bought a new fill valve and used the cap from it. Spent $10 or so too much, but got the job done. Another day I went to HD for something else and check on the caps. Guess what...yup, there they were. Terry's advice about the flapper is good, I had to replace mine some time ago. Flappers are cheap and easy to replace.