I have been asked to do some plumbing for a friend, and I'm stuck before I start. The first problem is the kitchen taps, which have been installed recently. It's a one handle tap with a built-in spray. Every time it is turned on, the spray hose starts vibrating and shaking. The spray itself works fine. The second problem is the bathtub taps. There is a good flow of water going into the tub, but when switched to the shower, no water or very little comes out of the showerhead. The problem is not in the showerhead itself, as that has been checked. They are on well water and so have a pump, which seems to be working OK. At least they seem to have lots of pressure in the rest of the house.
Sounds to me like the plumber or homeowner had a problem with soldering the joints. What happens is the solder beads on the inside of the joint causing an obstruction in the 3/8 supply line or ½ ” copper pipe cutting the pressure down. This is very common on single lever and shower diversion valves. Check it out by removing and re-installing.