Seems that SR is running low on memory.

Looking at htop it seems that Apache is the main thing to blame.
Seems that it has 3 main threads take up 112M, 768M & 702, totaling 1582m which is about 3/4ths of SR's total ram.

However they are only only actually using around 260M of ram.

One possible problem that's causing this is that SR has no allocated SWAP file or partition.