If it's a design fault, the only solution is a design change. You can raise the resonant frequency either by lightening the overhanging part of the lever, or by increasing the spring rate (how quickly the force increases when you bend the lever). If it resonates anywhere in the working speed range of the engine, it won't remain in a stable height-position.
Is the lever fairly long, and does it have a handle on the end of it? Shorten lever or remove handle - possibly both.
