Arent you using mercury
My last tune was done by one of the best in Sydney, it is good and the frist lot did an amazing job since they tuned it with an air leak the size of a 50c piece!
Not sure what is wrong, but it surges under load, maybe the clutch needs rebuilding (I bought it with not alot left on it), Maybe the turbo is broken or too small, Maybe the leaky gasket is causing it, maybe there is a restriction somwhere else in the exhasut. It is going to ba annoying that is for sure