Been grinding DRG levels with RDM as sub, and I tend to use some DoT or other as a pull, but I've been noticing lately that, with frequency, I'll get "interrupted" and then be "unable to cast" for a few seconds even though
- I am out of combat
- I have no damage-ticking status
- I have no aggro
- target is standing still, in range
- I am standing still
I swear it wasn't this bad grinding WAR with RDM. Is there something else in the game active-mechanical that causes this? E.g. too-heavy armor, 2-handed weapon, regen ticks, enemy status, weather, etc? Or is it just XI being XI and maybe dropping packets on the server's 56k telemodem?
Having a DoT on you won't ever interrupt spellcasting — only physical damage can interrupt.
Your issue is most likely movement related. There is a delay between client-side updates to position (i.e. when you see your character move or stop running) and server-side updates to position (i.e. where the server thinks you are).
If you start casting too soon after moving (before your server-side position updates), the game will have logged you as starting your cast in one position and finishing it in another. Because the positions don't match, you get interrupted. If you're lagging, the updates are slower and you'll notice the issue more often with spells/ranged attacks.
There is one silver lining to the way that movement-based interruption works: as long as you're stationary when you start casting, you can actually move a few steps right before the spell goes off without being interrupted (since your new client-side position hasn't been reflected on the server yet).
For spells with very long cast times, you can also run around (or get hit with knockback) in the middle of casting and avoid being interrupted as long as you return to the
exact position where you started casting the spell before it finishes.
(Edit: this technically also works for spells that cast more quickly, it's just more difficult to pull off when you have less time to run back) 
