I didn´t have those issues with the sauber i have to say. Another dependant factor is whether you drive the low downforce lm or the high downforce regular c9. Noboby ever said a 720bhp, twinturbo, 80s racecar would be easy to drive. I remember Jochen Mass, the Sauber c9 le mans winner 1989 saying in an interview that the sauber were twitchy and dangerous before they were up to temperature.
I think the way to get it round smoothly is not to Change too far down (rather stay in third rather than Change down to second, the car has enough torque to make up for it most times) and being easy on the throttle as Long as you are still turning. Another tip would be to check the boost. éverything over 85% can be hairy.
Edit: Tyre choice is all important. Use soft Slicks for a start if you don´t already.
I for example got the Impression the C9 has got much more grip even in the first lap than the xjr9 or the 962, which is odd because in assetto Corsa, the sauber and Porsche are the other way around. . I had ffb on raw, intensity on 80, Driver aids to authentic, Manual gearbox and i own a g29.