There are multiple reasons why you can't survive well enough in 60+ grifts when it comes to your character:
- You use prismatic armor instead of Force armor. At low level, prismatic is great but at 60+ force armor is much better.
- You lack some CDR (10% on the weapon, 8% on one ring), that means that you spend more time vulnerable out of your archon form
- You have no Arcane Power Regen / Crit (you can have +4AR/C on your left hand and +4AR/C on your helm), that means you run out of arcane power extremely fast and therefore cannot use your Zodiac ring at full potential.
- As you said also : There is an obvious scaling reason relative to paragons and gear. Your main stat has a huge impact on your sustain in general and i would not recommend to use the same setup as the top ladders that probably have 10.000 int more than you.
I would recommend to get some toughtness even if you lose some dps. I am currently P800 and farming lvl80 Grift solo and I am using Esoteric Alteration instead of Gogok. And i swear this is a decision i do not regret. You should try it.
Now concerning the dps variation : This is part of this build. Sometimes you will proc your Manald Heal tree times in less than 10 seconds, and sometimes it won't proc at all. There's nothing you can do except packing as many mobs as possible and hope for a proc