Sticks in third gear sometimes

Hello. Newbie here.
My car sticks in third gear occasionally. Not every time I drive it, but now and then when it is in fourth or just in cruising speed in town.
Recently, I had some odd occurrences with the electrical system. I cleaned the plugs, connections and terminals. On finding nothing wrong, I noticed that the frequency of the third gear shifting happened more often. As if it was getting more sensitive.
I don't hear any unusual noises from the transmission and other than the rpm's running faster than usual, it runs smoothly.
Thoughts? Questions?
-David