shower/tub control oscillates

We have a shower/tub with an American Standard one-handle control. The diverter control is in the valve mechanism, not in the tub spout. The valve is a balanced pressure type, and is called "Jasmine" in the American Standard literature. It has been installed about 3 years.
The problem is that when the button is pulled to direct water to the shower, it won't stay in place. Instead it oscillates in and out, making a lot of noise and providing very little water. I haven't taken it apart yet to see if anything is wrong inside. What should I look for and what parts am I likely to need to replace? I thought it might be a clogged shower head, but swapping it with another one didn't cure the problem.
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Go buy yourself a kit and rebuild the ENTIRE valve. Most of us in the field have learned over the years that it doesn't pay to do a partial re-build. A rebuild for your American Standard shouldn't cost more than $20. So rebuild the whole damn thing.
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