stortof It sounds like a memory leak. Especially with a 12 hour runtime, a small leak that is not normally noticeable might over time use enough memory that it causes problems.
If you use more and more CPU memory an app normally keeps running but slower and slower, as garbage collection takes more time and as eventually it starts paging out to disk.
In this case it sounds more like GPU memory. GPU memory when it runs out it stops allocating textures. Depending on your code this might cause an app to crash, or it might just fail to draw whatever uses that texture, or draw it wrong, untextured. Normally you have only 512MB of texture memory available.
This sounds like your problem. It is easy to verify: enable stats which has a line for GPU memory. If that is continually increasing over the hours it could indicate a memory leak. In which case you need to work out what is using GPU memory when it should not be.