When hitting 4000rpm in second I occasionally lose boost for a split second.. sometimes happens in first. It's completely random and doesn't always happen. Fuel cut or boost leak maybe? Or mapping issue?

is this all during full throttle? does the boost come back on after 4000rpm? do you have a boost gauge?

Under full throttle, it goes for split second then I have boost again. Yeah, I do 

Just in case anyone comes across this, it turned out to be the lamber sensor. I was quoted £350, so I recommend you do it yourself as it's like a half hour job, if that if you're more knowledgeable with cars. If I remember correctly, I paid £120 then like £15 for the tool.

Drove around the block, the car ran smoother, idled lower and pulled much better. I am, however, having issues with launching the car. It's not top of my list to sort since I rarely do it, but I will update this when I know the issue