A LOT of slowdowns and crashes - physics-related?

I've been experiencing a large number of full-blown sim crashes and even more of what I call "slowdowns" over the last few days, in Theretra and to a lesser extent in Caledon (I don't think I've crashed Caledon, honest, Mr Shang sir).

A "slowdown" here is what I call it when everything stops moving, I'm unable to move or speak, but I can still move the camera. Physical objects stop, particles stop, avs stop and so on but I can zoom around quite happily, and also turn my av. Sometimes this sorts itself out, but sometimes it ends with my being notified that the simulator is going down. The duration is usually thirty seconds to a minute, I'd say, though I've not timed it.

I don't find that this is repeatable absolutely, but I've only noticed it when testing a few new devices that use physics. None of these are particularly intensive. One is an autopiloted vehicle, with which I have seen slowdowns that fix themselves (I will be passing a point, everything will freeze, but after that things resume and the vehicle has moved on on its course in the intervening time, so I pop back at a different point, still sitting on it).

The other, in Theretra, is a weapon that rezzes multiple temp physical objects that have collision and land-collision events and particle effects - so nothing out of the ordinary for weapons, then. They're simple objects (just spheres) and simple scripts that play with llApplyImpulse and a few other things, and I've never had a problem with similar items before that rez far more objects. But I think the coincidence between firing a lot of things out and slowdowns and crashes is too much to ignore. I decided to stop with this project this afternoon and I've not had one problem so far, touch wood.

I've seen the "slowdown" before, in entirely unconnected situations, but only a couple of times since 1.9 (never before).

Anyway, just throwing this out for Lindens or anyone else. I'm finding it particularly frustrating as I'm trying to simulate gas clouds with the weapon which is a hard enough task as it is without the bloody sim going down every half hour.