Hmm, well a couple of things. First and foremost, how much memory do you have? Because the size of the MOLA data set on disk isn't necessarily what it requires in memory. It could be larger, and if you have less than 16GB of memory (or even if you do), you may have memory issues if you're trying to load the largest data set in entirety.
If you're already using the Geog Image Map then you may already have tested what I would think should work. Are you using the "Load multiple geog image map shaders" option?
I'll also say that very likely a majority of the Mars images you see are *not* using MOLA data but in fact a single spherically mapped displacement image, a number of which are available for free online. That's by far the easiest way to get a quick, realistic Mars, and although it won't hold up to closer shots, for orbital views it's generally sufficient. You can also spot-load higher detail MOLA or even HiRISE data to supplement in specific areas.
- Oshyan