My CH26 was finally diagnosed after fighting with it for three months, at two different dealers, and I ending up buying another mower because it was down so much. A third specialist was brought in and found a small cut in a wiring harness causing it, initially to ground out intermittently and run rough. But eventually got so bad that the mower would barely run.
Everything was thrown at it. Temp sensors, O2 sensors. Ended up just being a cut in the harness.