My tranny cooler lines weren't leaking at all, then the temps dropped, then I got the same thing you described: a few drops at the connection between the hard line and the nut on the flex hose. I just tighted the line a bit and the leak stopped.

The hard line fitting on my 300CD is some odd size, neither metric nor standard. I ended up using a large crescent wrench.

If you're going to replace, get new lines. 20+ year old used lines wouldn't be worth the labor to re-install them.