I've been successfully using the Canon 1.4X II extender with my Canon 500mm f/4 IS lens and Canon 7D body for the last six months or so. I purchased the extender new. Up until recently, I've had no serious problems with the lens/extender combination other than the inevitable slowing of the AF and a bit of focus hunting in low light. The image quality has been very good to excellent. Suddenly (over the last week, comprising about three shooting sessions), the images are almost all coming out soft. The problem is particularly bad when in AI Servo mode, but it also occurs in One-Shot. I removed the extender and the images from the bare lens are excellent, which leads me to suspect that the extender is no longer functioning properly. Other than trying to clean the contacts on the extender (which don't look at all dirty), is there any other troubleshooting I should do? And if the extender is indeed malfunctioning, what exactly can Canon do? It seems like a pretty simple device: no moving parts, no fancy electronics.

Thanks,

John