OK plug connectors on the coils replaced - stumbling at speed corrected, but unstable idle remains.

The old parts were in fairly good shape, and all plugs had a gap of ca .04". The electrodes were whitish, and all plugs were dry.

I read the code from the last stall, and had only a 10 ("Voltage too high at HFM").

I now think that I had a few things needing attention, but the whole HFM circuit was able to compensate until this mystery culprit failed a few weeks ago. ( I also fixed a vacuum leak, replaced O2 sensor and fuel pressure regulator). Up until the O2 sensor swap, I was getting 5's, 6,s, occasional 10, and 19.

Next diagnostic recommendation?

Thanks,
Tom