This all sounds like a CSU oil leak as happened to me a while back on an Arrow. The only reason I can think you would get an RPM reduction though is if the prop was generally at higher RPM than the lever position was commanding and every now and then the pressure was enough to cause the prop to move towards the commanded setting. I understand the 'unnerving' comment though. When it happened to me I was miles from anywhere somewhere over Brecon and limped back to Kemble, staying high and passing near Filton just in case something let go!