I wouldn't be so quick to assume the converter is bad. Many converters will have their output voltage sag a little bit under heavy load; they aren't perfectly regulated voltage sources, as that's not in the least bit required for this application.
The real question is whether the load is cycling for some reason, causing the converter's output to also vary, or whether the load is constant but the converter's output is varying undesirably. Neither one has been proven at all conclusively, from what I've seen, but it kind of seems to me like a varying load and a reasonably functioning converter.
