Had the same behavior on one of our CS:GO servers for over 9 months. Isolated it to the game not properly precaching the materials for env_ specific functions prior to creation.
To solve it we now create 1 of each of the following at the start of the map (OnMapStart) prior to any player connecting, to pre-cache the materials properly, and have not had the issue since:
env_lightdynamic
env_particlesmokegrenade
env_smokestack
env_steam (this is the only one we actually found to be causing the crash, but we pre-cache any of the env_ calls we use now, just as a precaution.)
This was where the random portion came in for us, as some maps have a steam feature built in to it (grates with steam, etc.,) so it wouldn't occur on every map, and it could take multiple hours to rounds to show as someone may not use a store item that called the env_steam function till all that time had passed.