Hey friend,
I'm kind of new and also have an older Darton. I askalmost the same question last year. After a year mine finally shoots ok. Here is what I did.
1. Went to a skinnier carbon arrow instead of the aluminum 6075 { The vanes are skinnier on carbons}
2. Bought a cable guard to keep the darn cables out of the way.
That helped a ton. But my arrows were still twisting their way to the target. The other day a man who shoots in tournaments watched me shoot. He found 3 more things tha thelped dramatically.
3. A stabilizer to help balance the front of the bow.
4. Talked me in to using a quick release
5. BIG ONE My nock was 1/2 inch to high causing the arrow to whip and dip. Now I get straight shooting arrows with a much tighter group.