'00 escort heater out/engine gague on HOT

Hi, My mom's mechanic cannot figure out the problem...and I'm really hoping someone here can! The last place to look at my car was the Ford dealership and it worked for a day and now it's finished again...below is an outline of work done/symptoms...thanks to all for looking and answering!!!
Symptoms: 1) Temperature gauge constantly goes back and forth from cold to extremely over heating.
2) The heater produces no heat.
Procedures:
Flushed heater core (3 times)
Flushed radiator hoses
Flushed radiator
New thermostat
New radiator cap
New Radiator
New radiator hoses
New thermostat
Tested for hole in head gasket – test negative
Heater core flushed again, this time at the dealer who explained to my mechanic that the heater core in the 2000 Ford Escort is positioned above the radiator and requires a special device to flush it properly.
After each procedure, the heat and temperature worked for a period of 20 to 100 miles, the latter distance with the most recent procedure (heater core flushed by Ford dealer); however, while the heat worked and the temperature gauge did not fluctuate so dramatically, it did continually fluctuate between cold and warm, and the temperature level fluctuated as well, but never quit working until yesterday afternoon. This morning, the car produced no heat and the gauge fluctuated from hot to cold.

An extremely similar problem occurred to someone on one of two other forums I visit, Taurus and Windstar, I'd say within the past month, so a search of them would be worthwhile. If things are slow I'll look. Right off, based on your symptoms and the FACT that your heater core is much higher than the rest of your cooling system, for whatever reason, you have an unusually stubborn, stuck large bubble of air in your heater core. Have they checked to see if the heater hoses are getting hot? If it is cold coming from the heater core to the water pump, then you have some sort of obstruction. Have they considered the water pump is no longer pumping at the pressure and volume it is supposed to? Another words, some Fords have had the blades on the pump erode through electrolysis to the point that they can't move the fluid like they should. Is the car overheating or running hotter than it should? Are you getting any CEL/codes? There is a second temp sensor that the computer uses and it would also detect these temp fluctations and would possibly cause operation problems, even to the extent the computer will be confused and have the trans run funny too. Have they tried running the car without the thermostat in place to see if the temperature guage at the very least won't go all over the place? Though it probably won't help with the trapped air if there is any. To get rid of that, the front of the car needs to be sufficiently high enough to make the core lower than the main part of the cooling system. Then hopefully the air will move out by bouyancy, but an opening needs to be made, like the radiator cap, to let it out, but the system needs to be filled when the air leaves or air will go right back when you lower the car. Well, I hope that''s enough to get you started. Good luck and let us know how things go.

Wow.....I'm not sure I know what all of that even means :) lol....I'm printing it off and taking it my mechanic. Thank you SO much for your time!!! I'll let you know how it goes

I agree with tripletdaddy. You've done everything but the water pump.