Here's a great video about mastering that helped my sound immensely.
https://www.youtube.com/watch?v=in46p78teVs&t=387s
The vsts in the vid are FREE downloads.
I also had a lot of that volume "flutter" in some of my older songs. It was the compressor in the Master FX chain "breathing" too hard, causing the overall sound to fluctuate with the hardest hitting sounds. This guy's mastering chain is very effective, yet still "transparent" - you don't FEEL the compressor working, it just does the job of leveling out the high and slow spots.
BTW - I like your tunes a lot - thematically they remind me a bit of Mithras (spacey metal), but your sound is different than theirs.