Temp controller. opamp triac

Hi all!
I built this circuit a while ago.. It works like a charm, but.... Sometimes the heating element doesn't switch on. I do get about 5 volts on pin 6 of the opamp. Checked for dry joints. Still the same thing. Its almost as if sometimes the triac fires on, and sometimes it doesn't.. Could it be the diode and 100ohm res? Or perhaps a too low supply voltage. Triac is a BT136. Feedback??

If it works intermittently I doubt it's the supply voltage, but you could try adding a reservoir cap across the zener to help hold the 5V supply up. I've had 1N4007s fail for no apparent reason. You could swap those out if you have any spares. If the 741 is socketed make sure it is fully seated.

Thanx Alec_t. will give it a shot.