Warming the keg up might help. Infections can also cause diacetyl. Letting the beer ferment longer the next time would certainly help...if it really is diacetyl. Are you certain it is? FWIW, I've had luck reducing diacetyl in kegged beer by krausening...adding a qt. or 2 of actively fermenting wort to the beer and letting the yeast chew through the diacetyl. Although at this point, I think I'd let it ride.