I think the fix was this (thanks to a post from Charles Dinges:

The solution was to re-position the line feed to the sensor. I had for sometime felt it was a wiring problem � not a knock sensor per se. Well, I was just standing there one day (pondering) looking at the knock sensor wire and where the wire was routed and suddenly I decided to move it away from proximity of the alternator. Well since then I have done my darndest to get the check engine light to come on � without luck. Yeah! Conclusion � my problem is solved. Shielding must be the important element here, so as to prevent spurious voltage signals from interrupting the signal from the knock sensor.