Does the water "rise" up a bit immediately when the toilet is flushed ? Does the water leave the bowl slower or faster than normal? Does it look like the water is being sucked out? I had a similar issue with a downstairs toilet - partial blockage on the pipe before it left the house... Seems like you may still have something similar..
Assuming all is ok with the pipe work, & no back up from outside pipes (can you check the manhole ??) then large amounts of hot water & flushing - see does that clear it... if not, then a plunger and plunge & plunge ... flush the toilet & plunge - you are going to get a bit wet & maybe some water will end up on the floor around the toilet, but keep plunging... when you think you are sorted out, then plunge another bit - just in case...
You can get "unblockers" to pour down the toilet but I found plunging the only solution to sort mine out long term...