Does your PC work ok when not running Arma? Does it ever crash with other games?
Are your fans all spinning?
Do you have good air flow clearance around the fans?
In Arma, do you get problems in both Single Player and Multiplayer, or just MP?
Have you defragged your HDD?
Check and verify that all the Arma game files are ok - Steam > Arma Properties > Local Files tab > Verify Integrity of Game Files
Run Arma with the public modset, play for a bit on the public server, and post your RPT to
www.hastebin.com so we can take a look for errors.
(your .rpt files should be in \users\yourname\AppData\Local\Arma3 )