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Some very good questions have been asked, but we haven't heard a reply as yet and answers would certainly help narrow down the source of your problem. Maybe add a new post in which all of the questions posed are answered then we can more easily analyze your situation.

BTW, have you tried opening an outside hose bib to check the flow/pressure there (a crude version of sammyg2's suggestion)? Your outside faucets are not tied in with the water heater and if they flow well, you can assume your supply lines (to the house) are good. If the exterior faucets also are weak, then the entire plumbing of the house is suspect.