When replacing gaskets on things like the oil pan and valve covers, you need to make sure you flatten out the mating surfaces on these sheet metal parts. This is especially true if someone (like the previous owner) was trying to get bad gaskets to stop leaking by continuously tightening the bolts. I know it's a pain, but if you didn't check this on your pan or valve covers, you may need to, especially if everyhting else checks out. Does the engine have an aluminum intake? If so, the rear seal on that would be a strong possibility. Sometimes people use the factory type rubber gasket on these, but really all you should use is a bead of RTV.
First thing I would do (if you haven't already) is to get the motor as clean as you can, then pay close attention whenever you drive it, to where the leak(s) are.