There could be many reasons for that - did you take a note of time and impacted app ids and checked server logs? If you have any intra-day reloads this could be impacting server performance. To determine what exactly is wrong I would start from checking the events logs and confirming the size of your server (simply to reassure it's big enough to handle all of your apps, concurrent users, data reloads etc). If this issue occurred for the first time 6 months ago it would be good to review any changes that were made to your environment around that time (new apps published? in memory caching issue? new users? new jobs running on your host?).