Set up: shimano 105; 10 speed; triple. When chain is on first 3 cogs (largest) and middle chain ring, it suddenly shifts to the small chain ring. This happens mainly when going uphill. I've tried adjusting the FD but still happens; not every time but always at an inconvenient time. Is it chain line problem? Or is the frame flexing too much? Or is the chain too short? Any insight will help Thanks