Peugeot 206 missing after heating up

ok hi there and hope you can help so this is the problem. its a 206 1400 great car really but the other week i was driveing it and it started to run lumpy as if it was missing from one cylinder and engine management light came on. turned it of checked coil pack connection sqirted wd in connections ect put in some feul system cleaner started up ran lumpy reved it it ran ok then a mile or two later started running lumpy again let it cool down changed plugs toped up oil as low changed coil pack for good one ran a lot further before the engine light started to flash again and it started to runn lumpy again so it seams that when it gets hot enough it starts to miss from one cylinder can anyone help its not overheating thermostat is ok the throttle body needs a clean as sooty as well as the stepper motor must be gunky someone said elctrical some say injector who knows