It sounds like a prop governor issue to me. A constant speed prop
will keep the RPM's constant until the engine power is reduced so
severely that the prop pitch has hit the limits. Surging RPM's on a
constant speed prop are usually the result of a bad prop governor or
fluctuating engine oil pressure (which the prop governor uses to
function). For example run the engine at 23" manifold pressure and
2400 RPM's. Reduce the manifold pressure to 20" and the RPM's will
remain constant. Continue reducing the engine power and eventually
when the engine is nearing idle the RPM's will start reducing (the
prop pitch has hit its limit of movement).