Cold water mixing into hot (again)

I've looked at all the cross-connect posts I can find, and still haven't solved my puzzler:

I discovered the problem when I tried to use the hot water in my new 2nd floor tub, through a brand-new fixture (not a mixer). Hot water for a minute or so, then cools down rapidly.
Same effect with the sink and shower in the same bathroom, and in the kitchen sink below it on the 1st floor (though it doesn't happen as quickly). This problem started in October, so weather's not a factor. (this autumn in Toronto was very mild)

The shower in the basement is a Moen mixer, and I get unlimited hot water on demand. Same for the lavatory sink and the sink in the basement kitchen.

The (rented) water heater was checked by our gas company tech.

I suspected the Moen in the basement, so I changed the valve, and when that didn't solve the problem, I installed a check valve on the hot line just before the fixture (yesterday). Still no effect.

I checked the washing machine and all mixers in the house by shutting off their supplies. No change.

I turned off the valve on the cold-water supply to the water heater, and got cold water coming from the hot supplies on all the fixtures.

I don't have a recirculating system, and I live in a semi-detached home.
Would a recirc system fix this?

Anyone have ANY ideas? I'm officially baffled now.

Dip tube scenario sounds like a possibility; was that pressure when you had the valve off at the water heater, was it diminished or full pressure?

If you did that test with a single handled faucet, cold can cross over to hot quite easily.

I mentioned the dip tube as a possibility, but the gas co. tech ruled that out.
I did do the test with a single-handle faucet, but the cold supply to it was shut off, so it could only get water from the hot side.

Upright_Animal said:

I discovered the problem when I tried to use the hot water in my new 2nd floor tub, through a brand-new fixture (not a mixer). Hot water for a minute or so, then cools down rapidly.
Same effect with the sink and shower in the same bathroom, and in the kitchen sink below it on the 1st floor (though it doesn't happen as quickly). This problem started in October, so weather's not a factor. (this autumn in Toronto was very mild)
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The shower in the basement is a Moen mixer, and I get unlimited hot water on demand. Same for the lavatory sink and the sink in the basement kitchen.
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I turned off the valve on the cold-water supply to the water heater, and got cold water coming from the hot supplies on all the fixtures.

Click to expand...

I think you have the answer here - there is a cross connection after the basement fixtures and before the upstairs ones...
Have you had any plumbing changes done recently (right before the problems started?)?

IMHO a re-circ system would not help...

No significant changes that would (I think) cause this problem. I ran new pipes for the shower and tub by branching off the existing lines, and they don't cross after that, and no mixer fixtures were used.

Where could the crossover possibly occur, outside mixer-type fixtures? If the hot and cold lines were actually connected to each other somewhere, wouldn't I have warm water everywhere after that point? What else should I be looking for?

Update:

Thanks for the replies.
I finally bit the bullet and started punching holes in the wall behind a basement bathroom. I found three lines visible: Cold, Hot, and one line that seems to mix the two. This third 'mix' line passes through the area I opened up, without connecting to any fixture in that bathroom, but by touching the pipes when fixtures are being used I can tell that water is passing through that mixed pipe any time *any* fixture uses hot water, and when about half of them use cold.
Is this some kind of anti-scald setup? If so, any advice on how to track down where it's broken, or where I can remove it?
I can post photos of the exposed pipes if that would help.