I can't figure this one out. When I turn on my shower/tub (single lever pull system), the pressure starts out great. Then, after 20 seconds or so, the pressure reduces by half. No other faucets have this issue. So I have already replaced the shower cartridge (didn't help), cleaned the shower/tub diverter (didn't help), and replaced my shower head (again, didn't help). I recently had my hot water heater replaced, but the pressure was fine for months afterwards. I also have a natural gas boiler/radiator heating system, but no water is being drawn to it. If anyone has thoughts on other possible causes and solutions, I would be very thankful.