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.
Sounds like you need to replace the mixing valve. This is a simple part to replace. This usually involves removing the handle and possibly the trim, then removing a retaining clip, and the pulling out the cartridge. Buy a new one and reverse the process.
Here's a good set of instructions for Moen valves: http://www.moen.com/consumer-support/installation-help/tutorial?id=moen0133
Best Answer
This sounds like an older style (washer type) installation. Is it possible that part of an old washer is stuck in the seat, or was pushed back behind the seat into the pipe, preventing water flow? First, make sure you have hot water TO the tub area. Turn the water off and remove the hot side, stem and all. Inspect for obstructions. You may need to briefly turn the water back on with the stem OUT, to blow out any obstructions. Have a helper turn the water on and back off, as you watch for any debris coming out. Once you have established hot water to the valve, you can check for an obstruction AFTER the hot water valve where it feeds into the tub spout, either part of an old washer or other debris. With the valve stem out, and the cold water valve off, try blowing air back through the spout to clear this (it will go into the hot water valve area) and then clear the valve out again before replacing the stem and washer, handles, etc.
Nine times out of ten, some obstruction will be in the line either before the valve or after it, where it feeds into the common pipe to the spout.