Hi Ellen- There are several reasons your tree might have started dropping sap but none of them will be easy to deal with. It sounds like your tree is older and many fir trees have been stressed by the long term drought we are going through and our hot summers. To compensate, they have been putting out extra cones which may be dropping sap for the first time.

A douglas fir will also drop sap if it has an injury to a limb or trunk. If you can see where the sap if coming from that might tell you what is going on.

And thirdly, there are a number of insects (I like your term "sap bugs") that can cause the tree to try and seal them out by producing sap. If you can find an insect causing the sap and get it identified we can talk further. This article has links to the common douglas fir pests.
https://extension.oregonstate.edu/forests/health-managment/douglas-fir-insect-pests

Your tree sounds like a big beautiful specimen, so if this doesn't help I recommend calling an ISA certified arborist to come take a look at it. Follow the search instructions at 

www.isa-arbor.com/For-the-Public

Good luck figuring this out!