It's quite odd that you've got surge on a CVH to be honest. I've only ever seen surge on a CVH when some idiot machined out the compressor housing and fitted a T4 compressor wheel inside of a .42 housing!

My car only surges once it gets up to about 1.85 bar on a GT28 but it's quite a big turbine housing without the RST elbow and a big compressor wheel, standard exhaust manifold, ported inlet, NMS head, 3" exhaust, GRS intercooler.

The only way you'll know for sure where the restriction is, is to monitor pressures around the engine. Try either side of the intercooler first.

It may be that your compressor wheel is now too big. A 2wd Cossie compressor wheel is a 57 trim opposed to the 4x wheel being a 60. Might be worth you trying that.