What else can it be?

I have a 1996 Mercury Sable overheating replaced thermostat and it is still doing same thing? I hit accelerator the other day and that is when it happened, it went from the middle of thermostat gauge to Hot instantly the coolant boiling and overflowing and afterwards could not get any heat coming from registers within the vehicle either. Went ahead and replaced the thermostat and it still doing the same thing, I had vehicle in idle mode let the car heat up after sitting cold the coolant bubbled eventually to the top of reservoir-we burped the air out of the car but the gauge is still in the hot area and the coolant is just going back into the engine. We are not sure what else it can be? Fans are running, not sure if there may be a blockage in water pump it does not seem to be. The Radiator blower motor is working took the car from my garage to the end of driveway and the needle starting going higher almost right to the Hot again. Not sure what to do can you please help?