buck regulator strange switching behaviour
Looks like it's in discontinuous conduction mode. This can be fixed by setting the value of the inductor to a greater one. This also means that the controlling loop (most likely) needs to be modified, unless it was already built for continuous conduction mode and you just donwsized the inductor. Here's a basic version in LTspice:



L's value is stepped to be 2\$\mu\$H, then 10\$\mu\$H. V(x) has oscillations when the inductor's current is zero (I(L1), red trace), vs the continuous, green trace. The two V(x) are shifted for better comparison.



DCM can also appear at low loads, e.g. you quote two examples: 5.7V@0.1A and 5.2V@15A. The first might be a low load that the low peaks of the inductor's current "reach" the ground.