Yamaha F115 idle speed dropping

[coconutkey]

I have a 2004 Yamaha F115 with 165 hours on it and it recently had the tps replaced by a Yamaha dealer due to another problem. When I got it back everything was right on spec and it started and idled right down to 700 RPM. Over the last 10-15 hours of use the idle speed has been dropping lower and lower. It now drops to 500 RPM after the motor has warmed up and occasionally stalls when slowing down from higher speeds. Restarts right away, but still idles very slow. I checked the ISC valve and found all three bolts loose. Tightening them made no change so I removed the ISC valve and cleaned it, although it looked pretty clean even before I went over it. O-ring was in place and appeared to be in good shape with no nicks or cracks. According to my manual there is no idle speed adjustment on this motor, but I'm thinking there must be some easy way to get the idle back to the 700 it should be. The motor runs fine otherwise. Am I missing something obvious?