TheLocalPlumber
I did not see anything in your post about a flushometer, so I will answer as if there is not one.
If you are taking a shower and the toilet flushes and decreases water pressure, then you could have old galvanized steel pipe delivering your water service.
The problem could be anywhere between the meter and the fixture, probably all the piping has decreased diameter and once determined to be a problem should be re-piped. Good Luck.
A test would be to install a pressure gauge at front hose bibb and flush toilet, at same time have someone watch to see if the pressure gauge drops.
The Local Plumber
Tustin, California
http://www.thelocalplumber.com
