I am playing a 2000 Collings OM-1A (25” scale) in DADGAD tuning. I am fretting the G string on the 5th fret with my index finger. That is a C note. At the same time, I am fretting the low D string at the 7th fret with my ring finger. Just the act of fretting this note causes a sympathetic vibration above the fret. If you were to play that note (above the fret), it would be a C also. The buzzing is really severe and causes the C note on the G string to really die out fast. I could record this if anyone is interested in hearing what I mean. Any thoughts?

These things happen, it's the nature of the beast. You might improve the situation by changing the neck relief. A little more or less. I would guess more, but that's just a wild-ass guess.