I don't know if you can help me here, the moment the weapon the streak is attached to, when it passes x coordinate 3700 (approx) the streak disappears, if I go back it appears again...Would you happen to have any insight into this? Thanks

Edit: I tried this:

vec_set(trail1.x,my.x);
vec_for_vertex(trailpos2.x,my,1); // vertex # for top of blade
vec_for_vertex(trailpos1.x,my,2); // vertex # for bottom of blade
vec_sub(trailpos2.x,my.x);
vec_sub(trailpos1.x,my.x);

And it kind works, stops the model disappearing, but then the interpolation code you implemented doesn't quite work the same, doesn't look as good...