Could be a whole host of reasons as to why this is occurring and you maybe best speaking to TAC in the first instance. What troubleshooting have you completed so far? What is occurring within your network between those times?
Depending on if the SSID is 'lost' verses poor speeds can lead to different troubleshooting methods.
In the first case, check the GUI or logs to confirm all APs are up and broadcasting. The at the same time, check the client logs, health and traffic.
If you review your AirWave data, this will provide a historical view as to what is occurring in your network in the given time frame.