Thanks Jasper. I am so frustrated with this problem - The root of the problem is this one f'ing toilet in the master bedroom. Upon advise from people like you I have done the following:
(1) Replaced the Fil Valve (The cylinder-like thing that connects to the float on top, and the rigid pipe leading to the shutoff valve).
(2) Plumber replaced the shutoff valve.
The problem went away temporarily for a day or so, but came back. What is this water valve you talking about??? I DONT want to rip open the wall, but this is the last aveneue I have, SO ANY HELP WILL BE MUCH APPRECIATED.