Sometimes the GM Customer Svc account here on the forums can help escalate a situation. They managed to get my car towed for free from the Wisconsin Dells down to the west Chicago suburbs where I live after it left me stranded two months after I bought it. The challenge in your situation is that they have to schedule an engineer to look at the car, which can be difficult because there aren't very many Voltec-trained engineers compared to all the other GM service technicians.