My advice is go in September - still hot but less busy as all the Parisians have returned to Paris. Go for a convertible, it will make the experience of being stuck in traffic so much more enjoyable - have a look at these guys who can rent you a convertible in Cannes. Driving in Cannes is not really frustrating, in my experience, traffic jams are not common and there are several car parks surrounding the city centre and orientation is relatively easy. It is most likely if you are hiring a luxury car, you will have access to some private parking - ask them as i am sure they will cater for this. The only time to really beware of driving there is during the film festival when it is crazy busy and roads are closed all over the city. Great time for car spotting though!
