Strange Stop Light Dash Warning Behavior

[hopz]

I have the 2001 X5 3.0 Sport... Three weeks ago I the dash board chimed and the indication was that the Left rear stop (brake) light was out. I checked. Sure enough it was.

I took the bulb out of the carrier and tested the bulb on the bench. Looked fine and the filament was not broken although the bulb has a slight dark spot on one side. Put bulb back in but still not working.

Went to NAPA and bought a two-pack of the bulbs. Replaced it. It worked. Dash warning is out. All was fine. Week later the Right side goes out while I was on a trip. Ha! I have the spare. Put it in and all is well again.

Day later the Left rear goes out again. (?) I was still on the road trip so I ignored it... several hundred miles down the road the dash warning goes away and the brake light works normally.

Next day it failed again- light on dash came back. This situation "fixed itself" again but it went out several hours later.

Bulb still looks new, and tests ok on the bench.

Ideas?