Similar thing happened to me with my SPF. Driving along fine, next thing the lights are weak and car is not running well. I too thought alternator or other wiring issue. Turns out that the positive battery terminal had come loose on the trunk mounted Optima. Re-tightened and has never been an issue again!
I also check the cables and connections to the frame and starter as well as other underhood wiring when I am under changing the
oil and detailing the car.