I assume that's 90F.
90C is waaaaayyyy too high.
Where are you located Mystro? What state?
I have been running 5S Zippy packs for ages & had no issues, apart from the usual MM related issues in the beginning which we all had. Since then, no probs at all with ESC, but, I have killed 3 of my 5 packs.
I've gone back to 2 x 2s 5000mAh 40C packs to make a 4S. Works awesomely!!
Can you say wheelies on command with a truggy?