I have a raelly weird problem with the toilet, it will work fine for a while, then all of a sudden it's completely blocked. I can then use the plunger and if I go really hard it will unblock and work fine for a while, then even if I only flush water a few times it will eventually re block itself. I can reuse the plunger really hard and it will unblock and again flushing nothing but water will get it completely blocked again, all other drains and the downstairs toilet works fine. When I use the plunger I rally need to push down violently with almost all my strenght for it to unblock or nothing will happen.

What could be causing this weird problem?

Thanks