Plumbing – Shower Runs out of hot water after a few minutes but sinks stay hot at the same time

Try pulling the cartridge out and turn on the water valve back on. Look to see if you have good flow coming from both hot and cold water holes. Doing this could blow out any debris in the lines restricting flow. Cartridges act weird when water pressure is not balanced between hot and cold.

Try turning off just cold supply and then just hot supply so 100% of water pressure is directed to clear out any debris.

Some showers have built-in valves for hot and cold. Adjust them so same flow is coming out of both holes.

Did you try adjusting the hot water safety limiter? It basicly just stops the handle from turning all the way to hot.

May I ask you to temper or clarify one adjective? Is this water literally BOILING hot (hotter than the tanks) or only SCALDING hot (as hot as the tanks, about 120*F)? If the tanks are also used for heating, then perhaps 180*F?

If the tanks are used both for heating and domestic hot water, then you have several large tempering valves, correct? These tempering valves mix hot and cold water together to achieve the industry-standard maximum 120*F for domestic hot water.

You're on city water, surely? High pressure, enough to push water to the top of the building, with pressure regulators on each floor?

This is the sort of thing that might happen if a tempering valve fails or is misadjusted, AND one or more of the tanks has developed a large air bubble. It's more likely to happen during periods when the municipal water supply pressure falls (because other nearby buildings are also using a lot of water). The hot-water tanks have been pressurized by high municipal pressure, including the one with the bubble, which makes it into a large surge tank. When municipal pressure falls, pressurized hot water backflows through the failed or misadjusted tempering valve into the cold-water system, where it's delivered to faucets, toilets, etc.

A really good plumber should be able to track down the source, but it'll take some time. They'd have to carefully measure the temperature of the pipes in the pipe chases, following the hottest pipes towards its hottest end until they come to the hottest spot in the cold-water system - that'd be the tempering valve in question. It'd be BEST if they could work while the occupants of the affected units were not in the building, so the water to their units would remain pretty static... making measurement easier.

The only other cause I can think of would be backflow (intentional or accidental) through a bathtub or laundry outlet, where hot and cold could be mixed without actually dispensing water - in the case of a bathtub (which has much larger water connections than a faucet or toilet), it'd require that the spout be blocked while both hot and cold were turned on. This would allow hot to backflow into the cold-water system.

I'm leaning very hard towards the tempering-valve problem, though, because otherwise we're talking about collaboration between numerous occupants in different units or a really odd set of coincidences.