Proc runinng slow once in a while
Is auto_update_stats_async on or off?

If this is off, it is possible that the query prepares, the stats are being discovered to be stale, and updated before the query executes.

https://docs.microsoft.com/en-us/sql/t-sql/statements/alter-database-transact-sql-set-options?view=sql-server-2017#auto_update_statistics_async