The new TL-LD1000 that I bought has an extra support to hold the batteries in place that my original taillight did not have. Before I ever got this new version though, I had an inkling that the batteries were shifting inside the light and causing a bad connection. My light had the same symptom of going dim after big bumps and just needing a small "adjustment" to the end cap to make it brighter. I added a small shim (some folded up paper) to hold the batteries in tighter and haven't had an issue since. Seeing this same fix on the new light confirmed my thinking about what was going on. Prior to the shim, I tried bending and cleaning the connections and a few different brands of batteries with the same bad results.

If my post is unclear, I can take some pictures when I get home today of my old and new lights showing the differences and my paper shim (it's so very impressive ).