Red Cedar tree dying

Lynda starts with ...
I have an old red cedar (at least 50 years old). Until spring last year it had been doing well - dropping & then leafing up again, flowering & seeding.
However, over the last 10 months it has dropped leaves from approx half the tree at th same time as the other 50% was sending out new shoots.The old bark on the main trunk & branches started to peel off & I noticed a spurt of growth on the old man's beard on the lower branches. Then the new shoots shrivelled up in the cold snap we had & they have not come back.
Now there are about 6 branches that look normal out of 20 - ie leafing up - but it does not look right.
Could it be root rot? Should I do a radical prune? Or just leave it & see what happens? I'm going to have to prune back at least 3 of the branches that are leafing as they growing over the roof, not a good idea in the upcoming bushfire season.
Please help!!
Lynda