The Case of the Phantom Lift Car

I've been building some stuff on the sea bed at the foot of the tower in Burroughs, and it occurred to me that there should be some way of getting down there to see it (all right, besides just falling in the water.  A dignified way.)  So I built a lift... you get in, touch it, the door closes, it descends (or, if it's already at the bottom, ascends.)  So far, so straightforward.

Then I added some visual detail.... a crane-like affair which hangs over the side of the tower platform to dangle the car on a sort of cable.  The problem now, of course, becomes one of how the cable is going to work when the car is in motion.  Eventually, I decided on a system involving two "cable" prims, one hanging off the crane, one rising out from the top of the elevator car.  When the car is at maximum height, the two overlap,  more or less seamlessly.  When it's at minimum, there's a gap between them, which I bridge with a particle stream - the sort of thing dubious types use for sending chains out to collars and suchlike.  Basically, a continual stream of particles is emitted from the lower length of cable - this being the one that moves - and targeted on the upper one.  Since the particles have the same texture as the cable prims themselves, the effect is, roughly, that of a continuous length of moving cable.  ("Roughly", because a) it's SL, and b) it's SL on my clapped-out machinery, so it's never going to look entirely consistent, is it?)

I decided to make the cables themselves flexi prims, for three reasons: it's a simple way of making them insubstantial (so they don't get in the way of things like sea monsters and submarines moving around the sim), it's a handy way to get the control handles at one end, which helps when you are pointing long narrow prims in various directions, and I figured I might want them to flex in the wind or bend under load or something like that.  So, I set these things flexi, admired my handiwork for a while...

... stepped into the thing, and fell straight through the floor.   The whole assembly, it turned out, had gone phantom.

Now, this might be one of those random glitches that make SL so much fun, and also so wet, but it might also be a genuine problem.  The thing which worries me is, the lift car uses keyframed motion to make its descent... and that requires it to use the prim-equivalency system, i.e. for some part of it at least to be set to "convex hull" or "none", instead of  "prim".  So what I am worried about is whether this is incompatible, on some level, with part of the build also being flexi - if the addition of the flexible cable was enough to confuse SL into making the whole thing insubstantial.  If so, you see, this has serious implications for my plans to ferry people around Burroughs on the back of a tame kraken....  More experimentation is needed, I fear.