How do I replace a 3-way lamp socket so that it doesn't die after 6 months? [solved]

I've replaced the 3-way socket in a table lamp about 4 times in the past 6 years. Each time it works for 6 months or so, but eventually the secondary contact that touches the "ring" on the base of the 3-way bulb (the low-intensity filament) starts arcing and the lamp flickers. This progresses to the point where the lamp flickers most of the time, at which point I replace the socket. Up until this last time the only sockets I could find locally were Leviton brand. Last weekend I found a Westinghouse brand socket at the local DIY box store.

Why does this happen? Anybody else have this problem? Will the Westinghouse 3-way socket have the same problem, and does anybody make a socket that will last longer?

EDIT: Adding some answers to good questions raised in posts below:

• Does the arcing problem happen with new bulbs?

Once it starts arcing it happens with any bulb. The contact for the low-wattage filament (the off-center contact) looks pitted and corroded.

• Is the lampshade (if any) attached to the bulb, and could be stressing the socket?

The lampshade is attached to a harp, there's no mechanical stress on the bulb.

• When it starts arcing, are the bulbs loose? If so, can they be tightened? Or are they screwed down all the way, but they wiggle laterally?

When a socket first starts flickering, tightening the bulb more tightly will quiet it for awhile, but that stops working eventually.