My understanding is that when your coolant is low in the radiator, an air bubble forms in the upper radiator hose - right were the water temperature sensor/gauge is. This air - much hotter than the water - causes the sensor/gauge to spike up, which causes the ECU to rev down the engine. Somehow this causes the air bubble to move away from the sensor and then the engine revs back up. But not soon after the air bubble returns and the engine revs down again. Again and again and again.
So just add coolant. Hopefully your car is not actually overheating. Does the water temp gauge needle spike up and down too?
My car was actually doing this, this morning. I haven't added coolant in a while and hopefully that's all I need to do. I seem to have to add collant every 6 months or so, yet I don't see a leak and I'm pretty sure I don't have a BHG.
Another problem is that many of these cars have a seeping head gasket, that is a blown head gasket that allows air to be forced into the cooling system. If this is the case then you have much bigger problems. If you have oil in your coolant and/or you're burning coolant (white smoke?) than you're up the creek.
I'm no expert in this, so if I'm wrong, please correct me.