When you mean shudder do you mean like at idle it wants to die/not catch itself on deceleration? Does the problem present itself under acceleration? If the car isn't throwing any codes and hasn't done it before it is possible that you just got a tank of bad gas.

Best way to determine if it's gas related is pull the fuel line off the rail and turn the car to on and drain the entire fuel system, including the tank and start over with known good gas