I have a long-established Cotinus coggyria that has started to
suffer from a very weird dieback. This affects some branches
(but only some of the shoots on them) and some stray shoots.
I tried cutting it back beyond the affected parts, but with
little effect.

http://i41.tinypic.com/15ckn7q.jpg
http://i43.tinypic.com/15s81g1.jpg
http://i40.tinypic.com/2i1msmd.jpg
http://i42.tinypic.com/b9br41.jpg

It is over 30 years old, and I think that it has been suffering
from the last few years' wetness. My inclination is to cut it
back hard, to neaten it up, and otherwise see what happens. But
it's annoying, as it is unsightly when it happens, which is usually
sometime in mid-summer.

Any ideas of what is going on?

Regards,
Nick Maclaren.