suddenly slow hot water throughout house?

14 year old electric hot water heater was on second floor. Hot water pressure is normal for first 15 seconds at washing machine (also second floor), bathroom and kitchen sinks and tub faucet (all downstairs) and then drops to half pressure. Flushed the tank, checked for leaks under the house and made sure cold water feed line is opened all the way by checking the valve, it didn't help. So I installed new hot water heater and still have the same problem. Thanks for your time!