Hi Meabh,
I recently purchased a 2010 Roadster and had the same P0133 error code come up within 100 miles of driving.... first the O2 sensor was changed, but the error still came up after about 20 miles. Currently back at the dealer to see if a solution can be found.......
Will let you know the outcome, soon I hope.
Incidentally I had experienced the inconsistent tick over... spooky
Regards
Mike
Perhaps a decent quality cat would resolve the problem. Seems to have helped mine a lot, at least as far as my nose can tell. The Omex system doesn't actually use the post cat sensor.