trying switching between memory and speed right before you render. if its on memory, switch to speed. and vice versa. youll find that it usually fixes it... it usually fixes it for me. also make sure your models are always at 0,0,0 and move the environment where you want them to be situated. the models can roam around the environment several hundred 100,500,800 etc without too much issue. its when you get into the thousands, issues arise.