I have worked put what the problem was.
The engine wasn't getting enough air Or was getting too much air at motorway speeds so
I removed part of the snorkel from the air box and now it's working fine. So for now I will only connect the snorkel when I'm off roading. It's only a 2 minute job to do up a jubilee clip.
Defo lack of air.what snorkel have you got. If its a diy job and you've used flexy duct,unless its reinforced with wire it will try sucking itself i sideout. Happened to me