What Don said is a very strong likelyhood, but I'll throw in a somewhat less likely prospet - a worn cam lobe. Once the hardened surface wears through the tip of the lobe wears down fairly quickly. This reduces the valve lift. At idle and low throttle the cylinder can still adequately breate through the diminished opening (better really - more velocity at low flow rates). But open the throttle up and that cylinder will suddenly be short of breath. Easy enough to check - just pull the valve cover and measure how far each valve goes down when fully open. The measurement doesn't matter - they just all need to be the same. Eventually the lobe will go completely flat and it will start missing at idle as well.