Great, I will try this.
Also, I don't know if this might be the cause or not, but I get warnings that I have undefined materials on some of my objects. The thing is, even when I assign a material to them, it still gives me the warning. It never seemed to cause any problems, at least not any immediate crashes. Could this be the cause, or just another symptom?

Any advice on assigning materials?

Is this enough?
my.material = metal_mtl;

Or is there some other process one must use?