First remove all the globes and clean the sockets with your special cleaning stick that you have now purchased!  Replace all globes with new ones if they are even remotely tarnished.  If still no joy, for the 'ground' side solder a copper wire along all of the bases and then run it to ground.  For the 'wire' side, try soldering the wire directly onto the middle of the lug - you may need a light file first and a fair bit of heat.  This will look a bit ugly but usually works.

cheers
Ian