Plumbing/toilet question

We have a washroom in our basement and today, after six months with no problems, the toilet clogged. I successfully got the gross stuff flush by carefully pouring a few buckets of water through, after which I did a lot of plunging. After several episodes of plunging, buckets of water, and a toilet auger, the toilet will successfully flush on alternate tries. In between the successful flushes a flush will fill the bowl to about 90% and then it will drain to about 40%. Then the next time I flush it flushes successfully and normally.

Occasionally, through this exercise, gurgling noises can be heard from the drain in the adjacent sink.

On one hand I am concerned that there might be a stoppage in the main line but our toilet on the second floor flushes normally and everything else (sinks etc) seems to drain properly. On the other hand, because everything else works properly and the toilet does flush half the time (and apparently normally), I wonder if there is actually a clog.

I would appreciate any information about this. Thanks

The vent pipe that goes up to the roof might be the problem. I had one clogged up by squirrels dropping walnuts down it. The toilet wouldn’t flush properly until we cleared it out. And put a screen over it so it wouldn’t happen again.

However unless the toilet has it’s own vent, you should be having problems with the upstairs toilet also.

basement toilets flush up. there is a pump involved, maybe in a small tank in the floor.

the toilet has to empty freely into this tank. the pump may operate every flush or when this tank reaches a certain volume. if the tank reaches the fill point to trigger the pump and the pump doesn’t start this could periodically block the flow (like every other flush).

Just because the toilet is in the basement dose not mean it has an up-flush toilet.
Those are more common to homes that have their own septic systems and the toilet is below the sanitary sewer outflow from the home.
Sometimes in these situations ya just gota roll up your sleeves and reach in and if that doesn’t do it, well go buy a new wax ring and remove the throne and have a look-see. OR call a plumber.
ETA; This time of year a vent may cover over with snow. Most don.t because of the venting stench will keep the snow back but you did claim this is a seldom used unit.

I think the variable behavior suggests it is an air vent blockage problem.

The air flow of the toilet flush is outward (The flush dropping toward the air vent’s junction) then inward. (the water falling beyond the air vent’s junction. )
The blockage gets pushed outward and then inward too… sometimes it seals quite well at the end of the travel and sometimes it doesn’t seal so much and lets more air flow past.