An Arma3 update (Marksmen DLC) broke a lot of things, mainly in regards to server stability and performance.
120 players on the server is almost unplayable, lots of desync, especially when more than 4-5 people are grouped together.
Lowering the player numbers seems to have helped a lot with the problems, but currently the ALUK devs are working with Bohemia to attempt to get a patch / fix released for all the recent issues. Maybe then, the slots can go back up to 120 / 140 players...