I agree, sounds like a venting issue. OR...if you have kids, it could be GI joe stuck in the trap.
Basics of venting...put a straw in your glassfull of water, Then put your finger over the top of the straw and remove the straw. The water stays in the straw. Remove your finger and the water exits the straw.
I had a problem of my vent pipe condensating and freezing on an outside wall in a 1840's home I used to own. The toilet would flush some days and not others, but the sink and tub would work. After I thought about it, it was because the tub had HOT water and the vent would defrost. Toilets only have warm. Anyway, that was my issue.