On my 2002 SRX I broke three valves over the past few years and had the blades fall in the cylinder. This happened even to a fairly new valve.
I was getting to the point of getting plugs built to replace the valves, clutch it like a ported Red Head 700 and just elliminate the power valves all together. A machinest friend hand built me 3 stainless steel power valves for me, not unlike what Polaris and Cat have done on some of their engines because of similar issues.
Hopefully the SS valves will elliminate the problem...ti me will tell!