Valve cover leaking oil, what's the dealio.

I took my valve cover off last weekend on the drivers side to paint it, anyway i put the cover back on and tightened all the bolts and i cranked it up and no leaks. But when you drive the car and stop and look under the hood i can see oil all over the lower intake by the egr valve. Today i loosend all the bolts and re-tightened all the bolts same deal, no leaks when i cranked up the car and even reved up the engine, but when you drive the car and pull over and pop the hood it's leaking again. What could be the problem, it keeps leaking in this one spot, could the gasket be bad, but how because when i retightened the bolts no leaks at start up, but leaks only when you drive the car. Is there more pressure build-up inside the valve covers when driving the car than at idle? Need some ideas how to stop this!!!!

Thanks for all the help on the subject. I figured i would go to autozone today and get a new set of gaskets, so after buying the gaskets i get home and take of the drivers side cover and then looked at the new ones. The new one's had like 14 or 15 holes for where the screws go through into the heads. The valve cover on our cars only has 11, man was i ever pissed, anyway luckily i still had the factory gasket and not cut it or tore it, put alll that $**** back together and drive back to autozone. I go inside and tell them they sold me the wrong part and that they wasted my time for nothing. So he looks up the part number on the box that they had sold me, come to find out the idiot sold me a set of gaskets for a 5.4 litre, what a retard. I told him 02 gt mustang 4.6 sohc auto, and still this duffis gets it wrong. So they credited my credit card, and the good news through all this when i reinstalled the factory gasket, no leaks, so this gu screwing up just saved me $40.00, so i was lucky after all.