I wouldn't say your situation is strange when you wouldn't say it was not present before. Speed and vibration fluctuations you are witnessing are nothing special at machines with changing load. And raw cement mills are definitelly in that group. When you are 100% sure it is a new phenomena, I would bet it is related to some motor's drop of power after overhaul and could be related to electric motor issues. I would also check if anything changed on the electric network this machine is connected to. Enough kV for the motor?