Are you refueling at the same station? What brand of gas are you using? Perhaps try a Shell or Esso fuel (non-ethanol) and see if it makes any difference.
Did you indicate that a tune has been done to the car already? Like, you have modifications that required a custom tune to change the way the ECU talks to the engine? If this is the case, you probably need some adjustments made for partial throttle applications. I've been through this as well, where it idles okay, and WOT (wide open throttle) is fine, but regular driveability needs adjustment. This is not uncommon with a dyno tune because the load is consistent, and the real world is not. It likely needs a laptop and a road tune to see what's going on, but it's probably a bit lean on the partial throttle. How do your exhaust tips look? Black carbon that comes off on your finger would indicate that its running fat (rich on the throttle), but typically these cars would surge instead of sputter. Lean would leave the exhaust pipes pretty clean, just the aforementioned sputter, like it is short of fuel.
