Where did the water pressure go....

My hose bib outside in the back reads @ 70-75 psi when the bib is turned on full with nothing on.When i turn on a sink on ,or flush the 1.6 gallon toilet the pressure drops to 20 psi, and it takes nearly 2 minutes for the tank to fill back up to water level. I thought right away it's due to the old water main to the house (galvanized pipe) all clogged up with years of minral deposits etc. The pipeing into the house tub/shower,toilet,lavatory,hot water heater and kitchen sink are all copper pipe. I had the city come out and check the meter and the water pressure on their end. All tested a ok. I told the city workers the problem and they said since the inside of your house has all copper we can safely guess there is no clogging as far as the copper goes.
As they got more interested in what it was they decided to run a video scope
in the galvanized water main...all of 65'.. They unhooked the water main where it enters the house.an the scope through. No major or even minor blockages just the standard scaly type stuff on the wall of the pipe..Very cool tool that video scope is!! in fact the scope went through the water main pipe very easily to the other end. Where should i look next !?

Thanks all!!

one other thing i need to add to this water pressure thread

The other area video scoped was the section of water main that runs under the slab to where the main threads into the run to the water meter by the curb. The under the slab run checked out clear as well.

Thanks again all!

Were they able to scope the main valve on your house? Often, a gate valve isn't all the way open or the gate is broken and partly closed. Since you have plenty of pressure it's obvious that you aren't getting volume somewhere. You might have a rock somewhere near your incoming line blocking a fitting.