After just riding 6 miles or so the three or four of my spokes in the rear wheel become so loose to wear they almost have unskrewed themselves from the nipples. I've hade the wheels trued by a bicycle shop but after 30 miles they started to do it again. I have an old 90's Diamondback aluminum mtb hardtail frame. Do you think it could be flexing too much. Would that loosen spokes? Otherwise I have no clue what the culprit could be.