The flapper can't seal totally so the slow leak cause the flush to cycle every half hour. I've tried several flappers but same problem

Try an epoxy putty on the inside of the tank. If it is a slow leak then this will be fine. If yu have a large crack then you need to replace the toilet as it is nolonger in safe working condition and you would not want to have it break when you are away from home.

Get an epoxy putty that can dry while still wet and roughen up the inside surface of the tank with a bit of sandpaper or steel wool before to give the epoxy a good surface to adhear to.

Hope this helps.