known issue, can be two things, carb icing(even in nice weather, not just winter), and wind shear across the ram air. If its bad in side winds, its wind shear, if its all the time, Id try a fuel additive, or super unleaded for a tankfull. see if it improves. Cheap supermarket fuel can be a beggar for it. Just one thing to check, ensure the ramair pressure pipe(the little black ones that plumb into the raimair ducts then(I think) possibly T together, and head back to the carbs to pressurise the cv chambers, are fully plumbed in, clipped on, and no leaks. If they are left off, the carbs refuse to work at speed, but if split slightly, they can lead to all manner of hunting and surging at slight speeds.