I would first plug your wheel to your computer and by using the Logitech Gaming Software, check to see if your wheel responds correctly. If this is your first time drifting with a 900 degree wheel, it's going to be quite a journey my friend. Practice makes perfect. You should check out the GT5 Drifting forum. They have pretty good advice as far as tuning setups and practice tips. Also watch onboard drift videos especially with the G27 and observe everything (coming into the corner, weight transfer, acceleration, wheel turns, hand brake, etc....). Turn off ALL assists with the exception of ABS 1. This might tie into the auto-correcting issue you're having.
Good luck buddy!