Wouldn't hurt to clean the contact points, any grease or oil there will seriously disrupt functionality.
My choice would be to clean/degrease with a proper solvent like Isopropyl Alcohol, and then use a "Fiberglass Brush" on brushes/contact points after...those things are amazing.
As far as correct voltage goes, i do have that data...
However personally i would not hesitate, to make a quick test with 4.5 or even 6 volt.
Too low voltage would display same symptoms as you describe.