had a friend describe just the same problem and solution to me today. In his case his Moen had shutoff screw valves on each side of the main valve. He ended up removing the screw valves and cleaning them out. Turns out that one was clogged and when the new cartridge was put in the lack of pressure on the one side caused the balancing valve to close nearly down, thus very low water flow. Maybe this might be your case also.