It has a fluid master type valve. Anyone know how to fix this?
I would suspect the fill valve if it just started doing it recently. If it's always done it, look into a blocked or non-existent vent.
Replace the fill valve, it's $10 and any asshole can do it in ten minutes.
Oh and also replace the flapper.
With fluid master full valves you can replace just the top half, shut off the angle stop pull up the ring, then just pull up and it will come right off. Then pop the other top on and push the lock ring down.
As Balls recommended, I would replace the flapper as well, just for practice if nothing else.
Balls Mahoney -Replace the fill valve, it's $10 and any asshole can do it in ten minutes.
I’m not just any asshole though.