Thanks for the reply, Mick. My ABS light never goes on except perhaps prior to starting. The idle symptoms I have seem different from those others have reported here, so if the zener in the OVP relay is NOT a regulator, I suspect it is ok. As a final test of the OVP, though, I'll check the OVP output voltage when the symptom is occuring - and when not. That should pinpoint any bad contacts in this device.
Opening up the thing is another story! Mine doesn't come apart when I bend the tabs back - the base is loose, but something seems to be holding on higher up. I thought maybe the mounting tab rivets go though an internal heatsink, and might have to be drilled out?
Another thread I found indicated many weird symptoms due to copper oxide deposits intermittantly shorting pins at various connectors. I'm beginning to have suspicions in that vein, since I have already found some past problems where that was a factor. I'll check all connectors that in some way could relate to the idle circuit, I guess.