W.W.
Yep - cooking the potatoes too long. They need to be on the up-side of al-dente.
I don't use red skin either. I use white or russet. Red potatoes are sweet (well, not "sweet" but not as bland as a white potato.
this is what I found on potatoes. http://www.potatogoodness.com/all-about-potatoes/potato-t...
What I have learned, what my mom taught me, was NEVER leave the boiling pot. EVER. Keep a knife handy and do NOT leave. LOL...
I don't use vinegar until the end either. Vinegar can break the potato down sooner, I believe.