Is anyone else having problems with their TLR-3 mounts, specifically on handgun rails?

I bought two over the last three years- put one on my AR, and one on my 2005-vintage .40 P99. The one on my AR seems to be staying put (for now), but the one on my P99 keeps flipping off under recoil. I finally cranked it down as hard as I could with that screwy coin-slot-only screwhead, and put it away for two weeks. Last night I pulled it out to go to the range and the light dropped right off- the plastic rail on the screwhead side failed.

Per the instruction I got with the unit, I'm using the "C" key. Just looking at the design, it failed exactly where I would have predicted (at the bottom of the vee molded into the plastic). Is this the reason everyone buys the metal TLR-1s and not the plastic -3s?

Hmmm...
I have one on a Saiga 12. No problems thus far, but all I've put through the gun since mounting the light is about 100 rounds of bird-shot.

You put it on TOO TIGHT.

the reason i got rid of my TLR3 was the battery, everything else about it seemed fine..

No issues with the TLR-3 on my pistol yet.

What sort of problems did you experience?

I guess by definition, it broke because I put too much tightening force on it and I did that because it kept flipping off the P99 rail.

The rail of the P99, wasn't damaged, either- it looks like the rail on the P99 is undersized. My P99 measures 0.820 edge-to-edge. My 3rd gen Glocks measure 0.835 (four samples) and the rails on my ARs measure 0.830 - 0.0832 (YHM, Daniel Defense, Troy).

If you look at the vee shapes on the TLR-3 jaws, there's a long ramp on the fixed side and a simple vee on the moving side. I think the sweet spot for maximum contact is probably in the 0.830+ range (following the spec, what a concept!). With an undersized rail, it probably doesn't have enough contact surface to fight recoil, and the light will eventually flip off. Or, you can overtighten it, get to to stick, and have the rail fail two weeks later

So the moral of the story- don't put the TLR-3 on an undersized rail. Streamlight does state in their instructions this light should work on a P99, with a particular slot lock block.

I wonder if I went "Red Green" and put a single thickness of electrical tape on the rail under the mount on a new one it might stay on..

To answer both questions, the battery life seemed shorter and more random in duration. I used genuine good batteries (can't remember brand but not cheap Chinese) and it seemed to just die sitting around turned off every couple batteries.

Also, the battery was hard to find in my area

That doesn't sound good.
I'll keep an eye on mine.