Have you had a coolant flush within the last 4-6 months? If so, try about 1/2 tube of the gold or silver dust stop leak. (Do not use the liquid pellets- rabbit turds) I had a leak in the same place about 3-years ago on my 94 LeSabre after flushing the cooling system. The silver dust stopped the leak and I have not seen a drop since! Believe it or not; they use stop leak at the factory to seal minor leaks. If you flush the system, leaks will appear. As far as valve cover gaskets leaking, you have no option except to replace them.
