To start diagnosis at a distance, it would be good if you could take digital photographs and email them to me. If possible, take photos of the whole tree, close up of dead branches. In particular, look closely at main stem at the point of origin of dead/dying branches, look for a zone of transition between healthy stem and any abrupt color change in the bark or signs of stem cankers. Take photos of that if present.
The next thing is to connect you with the Clackamas County Master Gardeners to continue the process of diagnosis. Best to use direct email and not the ask-an-expert system from this point on. If you cannot send photos, please email me and we go to plan "B".