IC thermal breakdown
I'll go out on a limb, and suggest that you look carefully at your heat sink. You label your 3 units A, B and C, and it's A which gives the problem. I'd guess that you have a single heat sink, and the three units line up on a common extrusion. My guess is that, at the A end of the extrusion, the mounting surface is not perfectly flat. So you're getting tight contact at one side of the IC, but not on the other. This gap is large enough that, even with thermal compound or a Silpad (and you are using one or the other, I hope) the thermal resistance across the gap is much too great.

The defect might be a slight protrusion around the mounting screw, or it might be a bend in the surface. Take a good straight-edge to the area in question, and verify that there is no distortion from a perfectly flat surface. Make sure to check in both axes. It might only take a few thousandths of an inch to be a problem, so be careful.