As a licensed plumber for 15 years, I have seen similar problems in apartment buildings.If the main water source is not fluctuating, it can sometimes be caused in individual units by replacing the shower mixer valve and adjusting it to compensate for an overpressure situation in either the hot or cold lines of the building.If that fails, then the problem is most likely a cross-over problem and your local health department would need to be contacted to correct any such situation.Cross-overs occur when persons in other apartments hook-up equipment to both the hot and cold water supplies but do not have proper check-valves installed or operational. This can happen if someone is using even a small portable washing machine, using photographic processing equipment especially since these have sensitive mixer valves that fluctuate easily, or even if someone is manufacturing something of an illicit nature, such as in methamphetamine production where they usually set up equipment to run all day or all night long, usually with just tubes and hoses connected to both hot and cold. When these crossovers happen, the hot and cold water push into the opposite line when pressures fluctuate, creating mixed water temperatures and unstable conditions.Only someone like your health department would have authority to enter and search for such activity, since it violates health codes for hot and cold to be intermixed like that. Also, if it is the result of any illegal business activities, you might consider contacting your local zoning authority, or in the case of narcotics manufacturing, your local narcotics enforcement agency.In my years of experience, once a cross-over is in place, it has to be found and removed before the apartment buildings water supplies will stabilize and temperatures can once again be adjusted to normal.Good luck.
