My XR8 is the same I managed to reduce it by cleaning the black ring of build up within the throttle body. Only other thing I can think of is that it's worse when I've used the cruise control.
__________________
Black Mac TE50 S3 NO: 106
Little Mac AU S2 XR8
Bite the snake before it bites you!
______________
_.__ /________...\
_.._(___\\___//___)
.._..(O...\____/...O)
|