Interesting - when do your tanks channel then? Blue and Purple? Our second regulator on orange tends to have 75-80% HP left, which is what I think is causing us to fail later on.
Our exact setup is:
Blue: Double DPS
Orange: Healer channel, highest ranged DPS middle
Purple: Tank channel, ranged DPS middle
Yellow: Healer channel, tank middle
It's an interesting setup that probably wouldn't work for every group, but one that has worked really well for ours. One of the biggest issues you would have with this setup is that tanks have to be really good at interrupts (especially yellow) so that the middle tank doesn't get knocked out, but with good placement missing one isn't a big deal. We've also had rare instances of a Regulator going lose onto a random Marauder instead of passing close enough to the middle for the yellow tank to taunt it, but it's very rare and there are ways to get around it. One way to handle it would also be having the purple tank "pass off" the Regulator to the tank in the middle with a taunt swap.
Having one Regulator still up with 75+% health is manageable once you really get the hang of it (and a few more pieces of gear), but it's a bit high for your first kill. It sounds like you aren't having much problem killing cores in time, so your DPS is definitely there, it's just about rearranging a few seconds in each phase to try and manage adds for your tanks and healers to have an easier time (and as a result, easier time for your DPS to mop up in those precious seconds before the boss is active).