sticking aperture blades?

[whitewash]

My sigma 18-50 f2.8 DC lens appears to have an issue where something goes awry, it doesnt appear to respond to the aperture setting from the camera and instead sticks in a particular place, meaning that images are over exposed.

im presuming ive got sticky blades as it only happens on this lens?

is there a fix for this or is a back to sigma for a repair?

jamie