Hot water pressure drops at kitchen sink
My kitchen is across the house and downstairs from the hot water heater. After the hot water runs for about half a minute and the water coming out of the tap is finally hot, the water pressure drops significantly. If I start the dishwasher, it becomes a trickle. If I rotate the handle to add more cold water into the mix, the pressure is back up to the level as before. My faucet is a Price Pfister 526-50XX with a single handle to control both temperatures. I have not witnessed a similar problem anywhere else in the house. I am guilty of not flushing sediments out of the hot water heater, too. When I have done it, I have had great difficulty closing the boiler valve and so decided that perhaps it is better not done. Where do I start to try to fix this or should I call a plumber?
Thanks,
Plantlady095