06 Mazda 3s 5 Speed Shudders
 

For the last day or so, my engine shudders or jitters angrily when I let off the accelerator at speed or when going down hill. From a dead stop, it seems a bit sluggish getting started, but this may just be my clutching. When accelerating, I do not notice this problem much at all.

The vehicle has almost 180k miles and has been well kept with regular oil changes and basic maintenance.

The owner's manual does not mention replacing the serpentine belt, so it has not been done and so I am wonder if it could be a timing related issue? It did start rather suddenly.

Any clues to source of my Mazda 3 blues?