Engine not perfoming well in hotter outdoor temps, 1.6 306 1998, -please help!

On my Peugeot 306 1.6 1998, I have noticed that when temperatures are hot
outside my engine isnt performing as well as it should do, as it runs very
different when temperature is cooler, got a lot more poke to it,
is this called engine pinging or something, as it sounds slightly different
as well, as i am unsure, and if so is there any way to cure this problem

Any ideas??
Much appreciated

 

Wouldn't be the extra work of the air con zapping the power by any chance?

 

No it couldnt be, as my air con is not working at the moment, lost some gas
pressure, and i also keep it switched off, must be something else, but i
dunno???

 

it's because cold air is denser and so holds more oxygen to burn with the
fuel.