Today whilst out I was giving it some stick in 2nd gear, as the power was coming in nicely and at about 3500 to 4000rpm I felt a hesitancy, it is like a couple of small jolts and at the same time there is a popping/snapping noise - it sounds like it is coming from the exhaust as in unburnt fuel igniting....not 100% on that though.

I tried gunning it a few more times and it kept doing to same, I am reluctant to keep my foot in as I dont want to do too much damage.

Any ideas what it is?

Couple of things to note...I have a Squadra chip and I am running it on RON95 fuel....worse still, it has a tank of Supermarket (Sainsburys to be precise) fuel in...is this of any significance - never used supermarket fuel before)

Thanks
Jack