Fermentation problem

[ethanol]

Here's a weird one which I have now had twice. Any ideas or help please?

I had a mash that I made up using malted barley plus white sugar and added Youngs Super Yeast and matching nutrient. S.g was 1.072. After a suprisingly quick three days the fermation had finished at 1.000

This afternoon I put it in my still and waited as usual for the vapor temperator to hit 76C which is where I start getting Ethanol at (thermometer is out a degree or two).

The temperature would not stop at 76C no matter how hard I tried to control the heat but went straight up to some 84C and then started putting out alcohol as the flame/spoon test told me.

It's as though all alcohols below 84C (the ethanol) have been missed by the fermentation and only the higher ones have survived.

As I say I have had this happen twice now, the first time was a beer brew that I wasn't happy with so put it in the pot.

Fermenting temperatures were held solid at 72 and the same yeast is fine in another batch.

What's going on?

TIA