The most common causes of frequent crashes and 'out of memory' errors are not having enough RAM, unneeded complexity in your model, and unnecessarily high render settings. There are several things you can do to avoid these problems.
1. Simplify your model by using primitives whenever possible. Reduce any unneeded complexity of Subdivision meshes, Boolean objects, Deformation lattices, Extruded objects, etc. Remove any objects that will not show in your final rendering, especially in files imported from other software applications.
2. Make sure you have made use of Shapes for repeating objects in your model.
3. Make sure the image maps in your textures are not higher resolution than is needed. In some cases, such as imported files, you can downsample the image maps substantially without seeing any degradation in image quality or in your final rendering.
4. Make sure to use appropriate render settings. You can check the Art and Science of Strata 3D and the User Guide for more information about render settings. In addition, the StrataCafe forums are a good source of information.
5. Do not copy and paste large images or models; this ties up memory in the clipboard. You can clear the clipboard's memory by copying a simple cube or sphere.
6. Make sure you have enough RAM. You may get an Out of Memory error if your rendering requires more memory then your computer has. Memory allocation is handled automatically by the program. It will request as much as it needs from the OS until there is no more to be given, and then you get the error message.
NOTE: Design 3D is a 32-bit program which limits the amount of RAM it can address to 4GB regardless of how much there may be in the computer. Subtracting system overhead gives a useable amount in the neighborhood of 3.5GB that the program can actually make use of.
7. You may want to consider upgrading your computer. The more processors you have available, the faster the rendering speed will be. So two, four or more CPUs will help more than anything. Design 3D CX 7 supports up to 24 "tasks;" a 12-core machine that is capable of hyperthreading equals 24 tasks.