I recently learned something that should have been evidient to me before... I thought..this is a good thing to bring up.
I've had some minor annoyances with my car since changing the battery and it seems to be coming down to corrosion at the battery cable ends. Specifically at the battery terminals, UNDER the boots that are put on the ends to give the cables a cleaner look.
The boots are not sealed and pull off when the bolt is out of the cable. Then and only then do you truly see what is underneath.
As a gearhead here on the forum, I've suggested cleaning the cables at all ends, etc to gain better connections and help cure electrical issues. I have never suggested pulling the boots and cleaning underneath as well.
Here is my example. I have chosen not to clean, but replace my cables.
These are pretty bad... Bad enough that when you are doing preventative maintenance or other work and have the battery disconnect. I suggest pulling the boots off the cables and looking to keep this from happening to you. Electrical problems can be very intermittant and annoying.