We live in a split-level house built in 1965; it has three residential levels plus a basement. Our bathrooms are on the upmost level, and the hot water heater is in the basement. The showers lose cold water whenever someone uses the kitchen sink, dishwasher, toilet, or bathroom sinks. The shower also loses hot water when hot water is turned on in any other fixtures. The problem does not appear to be related to the water heater, as we have recently replaced it (for other reasons). The plumbing and fixtures all seem to date from the original construction; we have changed only the shower heads to handheld models (separate hot and cold valves). Water pressure in the showers is not very strong either. Any thoughts? Thanks.