My advice would be to stop using DoTs and go with the 5 nuke setup:

Kromtus Lance or Kromtus Roar (Timer 11)
Kreig's Bite (Timer 6)
Frozen Miasma (Timer 14)
Visoracius' Maelstrom (Timer 15)
99 Kromrif Lance (Timer 7)

Put those on one hot key and try to keep all the spells on cool down.

Also, you can't put DoTs on a spam key or you will run yourself out of mana.

-Z-

Are you using Consumption of Spirit (beastlord version of canni)?

-Z-

Mana problems?
FPoS
Consumption of Spirit
PoS (another good click on named)
Tunic with Revitalizing Topor clickie
Miniature Horn of Unity on all three toons (another good click on named)
Bard mana song

...you must be trying to use all those DoTs.

-Z-

Ok... I attacked a mob and had my pet on it with taunt and the pet was holding aggro.  I hit DA and I was immediately tanking and the pet stopped attacking.  The pet did continue to taunt, but didn't do any damage to the mob until DA dropped.  I don't think this is viable if you want the pet to tank.

I would use the Companion's Aegis which is an AA rune you can apply every 30 seconds.  Companion's Blessing AA which can be applied every 12 minutes.  Fortify Companion AA can be applied every 15 minutes.  Mend Companion AA can be applied every 9 minutes.  You can also stack Aegis of Japac (Reuse 40s) and Auspice of Shadows (Reuse 50s) on your pet.  Make sure you have the epic 1.5/2.0 click buff on your pet as it reduces the damage they take.

-Z-

Just for posterity, and to update anyone else thinking of using their pet for tanking.

When I first started back, levelled to 105 and hit HH Sebilis, I had virtually no pet defensive aa's, and didn't understand what I was doing at all. I managed to figure a few things out and geared up in seb with 3 chars, a little bit at a time. By the end of HH, I had virtually 3 full sets of the glowing armour, and all the weapons that the bard, beastlord and berserker could use. My pet couldn't tank very much at all.

Now, my pet tanking aa's are maxed (except divine, cause I don't know much about that one), and with EM18 he is tough as hell. I am still gearing up a little with my 2 secondary chars (bard and zerker) and my pet, with summoned weapons and armour, can so far tank every named I've come up against in Chardok and Kor Sha Lab. I use the summoned thal plate/weapons that I buy from bazaar, because I find that if I don't have them I can get the occasional damage spike that kills him. I also need to use the agro weapons from the summoned bag of swords, because otherwise my rogue merc and berserker can out agro him on burns. Either way, pet tanking with 2 cleric mercs and 1 rogue merc works really well. I haven't tried it with 1 healer merc in a tough area yet. Might give it a go tonight, but for my current bunch of named ph's that I'm pulling (Sepulcher guy, Stone spiked guardian, Cutter, Screaming Tormentor), I don't need more dps, I need safety.

I do have a question though. What happens when I activate pet divine aura? Does he keep attacking / keep agro? Or do mobs see the invulnerability and switch targets? Not sure what the aa is used for really.