Close Couple toilet continues to run?

[IYBYISYL]

Hi, we have a reasonably new close coupled toilet(2 years old). It continues to run out of the pan sometimes. This isnt I dont think via the overflow(ive had a look in the cistern) ? I'm not familiar with the workings. I'm puzzled becasue it doesnt do it all the time. The lever sits on the top of the cistern (2 buttons). Anyone got any ideas why it should do this?
Thx
Jamie

 

[fitz1]

probably little square float catching on inside of cistern wall.

 

[gigz]

Next time it happens lift the lid off the cistern, if this stops it the pegs that push the flush mechanism are probably a touch too long and will need altering.

 

[TicklyT]

If lifting the lid doesn't stop it, and the water is below the overflow there may be a bit of grit under the flush valve seat stopping it from sealing.

I have seen a cable operated valve where the cable was binding, and a button that occasionally jammed a few millimetres short of 'closed' as well.