On the electrical front, keep in mind that things can be fine until a certain threshold is crossed, and then they don't work. Corrosion of the grounding wires would be an example. That's where I'd be looking - temp is not going to jump immediately because lights are turned on.
On the oil leak front, if the cover was not milled with the head, you will have a serious leak. Pull the top valve cover and gasket, and see if the top of the front upper cover is flush with the head. If there is a minor step you can try some silicone sealant over the seam and a fresh gasket. Make sure cover is torqued properly, starting from the middle and working out. Failing that, try Permatex "Right Stuff" gasket maker. I hear good things about it.