Water Flow Issues

Now that the house is about 7 years old, things are starting to break.
Both latches on the exterior doors need repairs, the nice wood deck built around the palm tree needs to be replaced due the trees expanding girth, all circuit boards have been replaced in every appliance right after the 5-year warranty expired, etc.

One of the most recent, and confusing, issues has to do with my water flow. If you remember in earlier blog posts, I wrote how the plumbing was designed so that I could turn on multiple showers, dishwasher, washing machine, etc., all at the same time and not notice any drop in water pressure. Well something has happened somewhere where only one shower can be on and the pressure is noticeably reduced. I have no idea if it is actually lower water pressure from the city due to the drought conditions in California or if my tankless water heater is clogged up. Does anyone have any ideas on how to go about checking this out? Please comment below if you do.

In hindsight, when the plumbing was put in, I should have installed a flow meter to help monitor water consumption. It would be a real easy test to check the flow if I had one installed right now.

UPDATE 1/22/15: Exactly one year later and I finally solved my water flow issues. It really didn't take that long but it got bad enough where I had to do something about it.
I unscrewed my 8" shower head and lo and behold it was clogged with about 1" of sediment at the filter. I rinsed it out and a few weeks later the problem came back. I went through this process a few times as I think the California drought is causing extra sediment to get into the water supply due to the low reservoirs. Also, the extra minerals are causing all kinds of hard water stains and clogging of the tiny orifices in the shower head. I took a Dremel and a wire brush attachment to polish off all the deposits. It worked great and now my pressure is back to normal.

Out of curiosity, I went to a faucet down in the laundry room where I can easily unscrew the filter. Guess what? An inch of sediment in there as well. I'm routinely going around the house and emptying sediment now about once every month or so.

So what do you do to fix this? I think I need to bite the bullet and put in a whole house soft water unit. UGH!