I think its probably the diff. You run the diff too tight and the car will spin on the entrance of the turns. Try loosing it just a little, and if it keeps doing it, loosen more and more. Remember if the spur gear slips at all, its too loose. I have no experience with the kit f104 tires. But I have heard they are not that great, but are ok on asphalt.
So what is the type of surfce you are running on?