Good that you mentioned the Breadboard, as it probably is the major contributors of noise in this case. Breadboard is a difficult environment for a switch-mode regulator, since loops and parasitic capacitances tend to be large, whatever the placement. Also, radial inductors tend to have relatively high stray field. I'd change that to something more shielded, but that is again difficult on a breadboard. However, I have many times soldered wires to a SMD-inductor and used it on a non-SMD-way in some occasions
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What is your regulator configuration, step-down or step-up? What is the input voltage/output voltage and current? For step-downs, my usual choice for casual use has been LM2675, it is very simple to use and it usually works satisfactorily even with non-perfect layouts. For step-ups, LM2588 (5 A switch current) or LM2585 (3 A switch current) might do the job.
If the regulator fails to solve the problem, you might build the regulator on a separate copper clad board (using Jim Williams high-speed style
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), using it as a ground plane, and just connect this "sub-board" to breadboard using wires. This will eliminate ground-induced noise.
Regards,
Janne