volume question

I have a 3000 sq ft home that when we flush a toilet on one side of the house, or run the washing machine ,there is a major pressure loss everywhere else. I checked the water pressure on one of the garden hose faucets and it is around 60 psi . I assume that there is a volume problem. The house is around 20 years old and in Gold River , ca. Alot of my neighbors had line breaks and had to take the plumbing overhead because the builder buit this addition (2500 homes) on a bed of cobble.
It has gotten worse since they change water service and installed new water meters.
There are no asbuil;ts for this community. It is an unincorporated area of Sacramento County , which obviously the builder got away with anything. I went to the county who told me only the architect would have the plans . I went to the Architect and he said it was too long ago and he dosen't have plans and that the buider changed his mind so much during construction that lord knows what is under my house.
Certainly would of like to have known this fact before I bought but now I am stuck.
Can anyone tell me how to proceed in checking out why there is such pressure loss just by flushing one toilet.

FIrst, make sure that the shutoff to the house is fully open. Then, if you have a PRV, it may have failed. Then, take a look where the water comes into the house and try to determine what kind of pipe it is, and what size it is. Then, get back with your observations. A bad prv can show that, but you may or may not have one. Galvanized piping eventually rusts and can choke the volume off prior to rusting through, so if there's any of that, that may be the issue...all it takes is a small section to do that.

You need to check the pressure where the line enters the house WHILE the water is flowing in one of the faucets.