Like I said, check the pushpins are fully home, they can be a pain to seat fully, use the tip of a pencil (one with an eraser on it) to push down on the cooler legs, they should be in solid contact with the motherboard, if you can feel, or see any 'spring' in them, they're not fully home.
Liquid...Meh...I've played with it but concluded a decent air cooler is easier to live with.
At stock speeds you don't need anything special anyway, the little Coolermaster Hyper 212 Evo will do very nicely.