Toilet that flushes and then doesn't

[Really Stumped]

We've been in our house almost 32 years . We have a basement bathroom with shower, sink, and toilet.
The toilet is a little over a year old.
Up until about 2 years ago, everything worked fine. Then the toilet started to not always flush.
We snaked lines, ran cameras... no problems.
So we installed a new toilet.
Same thing. It will flush 20 times, then won't.
Wait awhile, it will flush fine again.
Then it happens again.
Nothing in this bathroom has ever been vented, so we thought OK, we'll put in a studor vent that ties the toilet and sink together.
Keep in mind, for 30 years, never had a problem...
But this on again, off again lack of flushing is a problem.
So, we had a studor vent installed by a plumber.
Toilet worked fine, then wouldn't flush. Same thing.
So we tore up the bathroom floor, installed a tee under the toilet. The waste goes out the front. We installed another studor vent for this toilet. This new Studor vent is in the back.
The toilet won't flush with studor vent on it, but will if removed.
We're really stumped.
Why issues after 30 years?
All outside sewer lines are clear, too.
No standing water.
Less than 3 feet away is the studor vent for sink and toilet that plumber installed.

 

[Reach4]

You need a roof vent somewhere in your system to keep the pipes from being pressurized. What is the nearest thing that has a real non-AAV vent?

If you put a piece of plastic over the open vent, and flush, I suspect you will see it is blowing pressure.

 

[Sponsor]

 

[Jadnashua]

If there is an obstruction in the drain line somewhere, depending on what else is going on in the house, the line could be full, and then, the toilet won't flush.

Describe what you mean by won't flush, and what happens.

Does it fill up then, slowly drain out without gaining enough velocity to carry away solids?

Does it just not expel the waste, but otherwise seems fairly normal?

When the toilet won't flush, does anything else drain slowly or not at all?

Do any drains make any burping sounds like trapped air bubbling through the trap?

You indicated you replaced the toilet, so it probably is not the toilet itself, but something in the drain line.

How far did they run the camera? There could be a section of the drain line that has sunk, or it is broken (any heavy vehicles been driven across the area where the drain line goes?). If there's a slow leak, and there are any trees, they may have filled up a section of the line with roots...so, verifying the line is clear all the way to the sewer connection may be called for. A 4" or larger pipe can hold a fair amount, so things could go normally unless you fill it up faster than it can drain. Does it only fail after say the shower has been run a long time, the washing machine has run a load or two, or some other thing that may fill the line up?

There can be various things that could cause this, but it appears it's not the toilet itself. A bit more info on the sequence of things may help isolate the problem.

 

[FullySprinklered]

You may have more than one problem. I don't know what problem #2 is, but problem # 1 is that you have a toilet flushing into a tee lying on it's back. If I understand that the lavatory has a vent of it's own, you don't need that rig running past the toilet and turning up into the sky if that's your vent of choice. Best place for a 30-year-old toilet is in the back of a truck headed for the landfill. From your post, it could be that problem #2 is somewhere downstream.