Anne...
I'm no expert, soo...
The first thing that comes to mind is humidity. Given the age of the frame, I would expect the timbers to have acclimated to ambient humidity by now. Assuming of course that the sealeant is not doing an incredible job of holding in the moisture. A pin type moisture meter will quickly confirm if this is the case.
I would not be surprised if the relative humidity of the house in the summer is the cause of the problem. A relative humidity meter will make fast work in confirming if this is the case. If it is, I think increasing air flow through the home MIGHT help, by AC might be needed.
Other possibilities: If part of the living space is below ground, the cool floor and walls would very likely condense water from the "fresh outside air". Conventional wisdom says open the windows...and "air out" basements in warm weather. I think conventional wisdom in the case is all wrong. Warm outside air holds a relatively large amount of water. Bring this "fresh air" into a cool living area and the RELATIVE humidity will increase. Mold and such love such an environment.
Moisture seeping through concrete walls and floors are another possible source of moisture.
Anyway, to boil this down to fewer words: There is moisture. Determine the source of the moisture. It's coming from the timbers, or somewhere else. Test timbers with pin type moisture meter and ambient inside air withe relative hudidity meter.
Hope this wordy reply is of some value to you. Let me know how it turns out.
Joel