Toilet leaking during flush

[Newell Attack]

I am having a leak occur during the flushing of our toilet. It looks and feels like it is coming from the valve (of course I am looking in a mirror because of the location of the valve). I had this problem a year or so ago and I took the valve apart an checked it. There didn't seem to be anything wrong so I put it back together and the problem disappeared. Now it has reappeared and I was wondering if anyone else has had anything like this happen before? Could I have run into a surge pressure of some sort? Any inputs would greatly be appreciated. Thanks