Water pressure issues

I've been staying at a friend's house, and it is suffering from water pressure issues I can't figure out.

At first I thought it was the shower valve.  Then I realized its the whole house.

The house is actually a nice late model mobile home in a rural community.  It has city water.  It used to have excellent pressure at all fixtures.  Now it dribbles at the shower, and you can't run 2 fixtures at once.  There are no devices in line like water softeners, etc.  The house is plumbed with PEX.

The city water lines were upgraded about a year ago.  The old 4" water main was upgraded to a 6".  Everything worked fine after the work, but the issues have developed since.  The city says they don't have any line pressure issues.

My initial thought was there is a partial clog somewhere, probably caused by crap that was knocked loosed during the upgrade.  The problem is that the water doesn't seem to behave like is clogged.  I would expect a clogged water line to have better pressure when it was first turned on after a period of non-use, then slow down to a trickle when the clog prevented enough water from passing through.  It does the opposite.  When you first turn it on in the morning, its a slow drip which will build to a moderate ( but still inadequate) flow after letting it run for a couple minutes.

I'm a little stumped where to begin.  Thoughts?

At the beginning...

Is there any sort of drain valve on the underside of the home?  I'd go under and see what is there.  Line out of the ground must come up to a shut off valve, or to the water meter.  

E36 M3, I don't know.  I always hesitate to loosen fittings on pressurized lines, but I'd be tempted to crack the inlet and outlet lines on the meter and compare how wet I got, and how quickly.

You are right feeling its odd to not have a burst of pressure on first opening the faucet, if it's only a restriction.  Could there be some sort of pressure regulator somewhere?  Never seen one on water.

In reply to Streetwiseguy :

I'm searching for a pressure regulator now

Rig up a pressure gauge to attach to the shower pipe, then move back. Bathroom sink pressure, back to hose pressure. See if there is a drastic difference anywhere to point out where a restriction might be? 

I wonder if there isnt a forgotten filter somewhere. Its getting clogged more and more and lowering flow. Also that would jive with the construction work. That would have introduced some debris. 

I may have found the problem

Im pretty sure that's the top half of a 50 lb pressure regulator. I think it would be pretty hard to adjust the pressure if the pressure regulator is blown apart

My old house in Virginia had a water pressure regulator where the main came through the slab.  That had a port for installing a pressure gauge post regulator.  Maybe a messed up regulator could be slow to respond to pressure changes.

The only leak I had was the regulator, of course while on vacation for a week.

No leaks.  

I feel kind of smart for first thing on a Sunday to have sprayed out a bunch of answers, and one of them was correct.  That doesn't always happen.