Insert statement sometimes runs slow and sometimes fast SQL Server 2014
You may have a disk bottleneck.  Run a Perfmon.exe recording specifically you want the individual "average disk read wait queue" and "average disk write wait queue" of all drives.  Perfmon is part of windows functionality and is on all Windows PC / servers / etc.  You will want a recording from a weekday and a recording from the weekend.
Are you running DBCC database integrity check on weekday nights?  While I want this running every night, it causes to much performance problems so to live in reality, mine only runs on weekends.