When I stop at a light, my car shudders?

Question: I have a 2000 Jaguar S type V8 and When I stop at a light, my car shudders, What could be the cause? Could gas be the problem. I''m using medium grade. Also, I just had a tune up last week when I replaced all four tires. Answer: Was this problem there before you had the tune up ?? if not, I think one of the plug wires probably did not get fastened securely. Response: I took it back to the shop that did the tune up and they found a wire had come off. Thanks for all your help!!