I've have a 2009 Astra vxr for a year now at 66k miles.

In the past month I've had a problem when starting where the Rev counter would bounce from 600-1500, the engine would rock and the exhaust chug, with the vibrations felt through the accelerator.

The problem is that bad it can clearly be heard from inside the house and is starting to get embarrassing now as well as being an issue that I can't just get in my car and drive off, I have to keep turning it off or waiting for the problem to stop before I can set off....

Any ideas where to start???

Car has been modified (remapped, miltec etc) before I bought last year but I've had no issues up until now (other than the gearbox refurb last year!)