The S11 boxes are sometimes affected by bad capacitors in the PSU, which can start off by causing random faults before the box dies completely.

It may be worth taking the lid off & having a look at C11 (1000uF 10v) & C15 (470uF 10v) on the PSU board. The tops of the capacitors should be perfectly flat. If they are bulging, or if any brown gunge is leaking from the top or bottom, then they need replacing.