Hesitation when accelerating after gear change, goes away with battery disconnect

[BoxsterInTheSun]

I have a slight hesitation when first accelerating after a gear change, it lasts about 1 second, then proceeds as normal. The interesting thing is that if I disconnect the battery overnight, the hesitation is gone and the car runs perfectly for a while ( maybe 200 miles? ), before the hesitation eventually comes back.

There are no codes and no check engine light before or after.

Any ideas would be gratefully recieved, it's kind of driving me nuts.