gas furnace, burners wont stay on
 

Hello everyone. I am having a problem with my gas heat in my house. Two days ago my heat stopped working, completely out of the blue. I went into my crawl space to check it out, and I havent been able to fix it completely yet. My pilot light never goes out, but the burners only light if I press down the "reset" button on my thermocouple. I normally only have to press the button when I am doing my initial lighting procedures at the beginning of the season. Once I press the button, the burners light up, the fan will come on, and voila I have heat! But as soon as my inside temp reaches the desired temp, the burners cut off and will not come back on until I go back into my crawlspace and hit that darn button again. Any Ideas? I was thinking it was probably a bad thermocouple, but Im not exactly sure. The unit is probably about 25 years old, natural gas forced air heat, and the type where the pilot light is always burning.
Any help would be greatly appreciated.
-Mike

Is that hook up to a Johnston control thermo control safety box? Note are you having to relight pilot?

The pilot always stays on, it never extinguishes

Is that hook up to a Johnston control thermo control safety box?