Okay so after taking in all the advice listed above, I think i'm almost there!
So working through tommy's list this is what I did.
Firstly, I turned down the amps on the everlast plasma cutter from 42 amps to 35 and slowed down my cut feedrate. I thought this might help with power fluctuations.
Secondly, I went out and got another good 2 stage air filter, giving me 4 in total now. I am also toying with the idea of adding my own homemade wall mounted desiccant filter, does anyone have any experience or recommendations with this?
Thirdly, I double checked every wire to see if there were any broken wires.
Next I played around with the cut speed and I ended up dropping it about 40% (seems large but I also dropped the amps too)
After all this, and Rod's new arc ok resistor resistor blowing away the built up oxide, I am getting reliable cuts with about 30 minutes of cut time not throwing up a single error yet.
Thanks for all your help guys and fingers crossed I dont have to reply to this topic again!