My 96 gets hot and idles lower than normal

[96markviii]

I own a 96 Lincoln Mark VIII. I have had it for about a year and it ran perfect until now. First the heater would not work and temp gauge read that it was hot, the needle would go all the way up then it would go back to normal and the heater would work. I changed the thermostat but the old one was fine. I checked the radiator it was clean with no clogs or leaks. Now it still gets hot and the idle is at about 5-600 rpms opposed to the 1000-1200 rpms it was at before this happend. I was told that it could be the water pump or the throttle positoning sensor or the heater core. I am not a rich person so I cant go out and buy all of these parts at once to see what it is. Someone please help!!!!