My toilet stops filling. Fluidmaster fill valve currently installed. The sink on the same line runs just fine. If I turn off the water supply to the line, and open up the cold water of the sink all the way, I will hear a little hiss from the toilet fill valve. Next I turn on the supply a little, and keep the cold water in the sink running, and the toilet will fill. If I turn off the cold water in the sink or turn the water supply on too much the toilet stops filling. What's wrong?

So you have a FluidMaster ball cock. This is a common problem with these ballcocks. They have a small washer with a hole,(see image) in the center that swells shut in time preventing water from entering the tank.
Purchase a FluidMaster Seal#242 from your local big box store. To replace it you will have to shut the water off to the tank and take the tank lid off. Twist the top of the ballcock off counterclockwise, (see image) and pull up. Pry out the seal and replace it. Reassemble, turn the water back on and give it a flush. Better now? Regards, Tom