Euphorbia Species, Silver Thicket

Hi there,
Wondering if anyone might be able to offer some advice . I bought a Silver Thicket about 3 months ago. Its about 6 feet ...Read Moretall and beautiful, I believe nearly 40yrs old. I live in Northern Michigan so not the sunniest place but I put it in a Northwest Window where it gets lots of afternoon sun. I've only watered it a few times. About a few weeks ago I noticed a couple of the bottom branches grey and dead and then several more so I trimmed them off. A few more died again so I repotted it . I was worried that it may have root rot from possibly sitting in water but roots were dry. Today another branch died over night. Gave it a water. I'm worried that it is sick. Any suggestions. Is it just molting. Would hate to loose this tree. Not much on the intranet about it.

This is a really curious but dangerous plant. It has a nice blue-green coloration with very sharp but thick, succulent spines. The new ...Read Moregrowth is more green and soft to the touch, but hardens in a few months. Eventually grows into a tree with a gnarled, woody stem, topped with a globe of spiny, blue 'leaves'.

Freeze of Jan 07 showed that this is not a very freeze tolerant plant... though mild frosts never seem to harm it, temps into the high to mid 20s definitely kill plant tissue. Lost 2 seedlings about 2' tall from temps around 26-27F.