HELP!! My water pressure- comes and goes away completely when faucets are turned on, and if you wait another 5 minutes the same thing happens again- The owner turned the main water valve off before he vacated the house we are renting and we had to find valve(s) (there are two) I didn't know which one is which so I turned them both on (counterclockwise). Now I have water pressure problems- it starts great then it goes away- what could cause this?? What is the second valve for??
Thank you

It could be that the main water valve is not opening all the way. In some old, seldom used water valves, yoke gets disengaged and gets stuck half way inside the valve. Valve may feel it is opening all the way but in reality it is not.

Also, you could have faulty water pressure regulator. If you have hose valve before the regulator, turn it ON and see if you get good continuous water flow.

In both cases, you will probably have to call a plumber ( or handyman ) to attend the problem.

Are you on well water or municipal water?