I had my boosted gsr eg tuned a while back and was only able to put down about 330 whp at around 8500 rpm (cant remember exact) because the s300 would quit out of data logging when rpms were high. I have proper grounds and ngk bkr6e plugs. Ngk spark plug wires with stock dizzy. The wire harmess is tucked and some extended wires. Battery is relocated to truck. What could possible cause so much interference.

 

Make sure you are using the usb cable with the nanofarad (bulg in the cable).
Make sure your engine/tranny ground is good.
Try another laptop too.

Your tuner could have used on board datalogging to finish the tuning if he wasn't capable of tuning without datalogging or live connection to the ecu...

 

I believe thats what he did (im not that familur with tuning s300). He said he would have to keep manually logging (if that makes sense) so it was taking alot longer then it should have.