Toyota Repair: Heater only blows warm when accelerating, coolant loss, toyota 4runner

Question
Hello,

I have a 1995 Toyota 4Runner and the heater is only blowing warm air when I step on the gas. Since this problem has developed, I have also noticed that the temperature gauge doesn't just gradually move to the middle anymore. It stays on cold and all of a sudden jumps to the middle where it is safe. It's not getting hot, it's just not warming up like it used to. I also noticed, that until the gauge reads warm, my engine idles really high. I have been told that it is probably my thermostat. While investigating, I noticed two little cracks on top of my radiator. Would that cause this problem also? Is it more likely a combo of the two? Thanks for any help!

Answer
The erratic temp gauge behavior certainly would indicate a thermostat sticking so I would recommend replacing it, the cracks on the radiator would not cause the problem but I would be concerned because of coolant loss which can lead to overheating so I recommend replacing the radiator also.