This is 1987 OBD1, don't think is has a code for misfire does it??
Regarding the connections, yes - I did a continuity check on the leads including the O2 sensor ground from the ECM to the engine block and all is fine.
The signal from the O2 sensor starts off fine and consistently goes bad after 10 minutes so what else could be causing that repetitive pattern? I don't know what else to check other than a leakage of air through the engine to the sensor - getting pretty confused about this by now, even more so than normal
What about that exhaust pipe leak I mentioned - I'm going to be fixing that right away anyway so guess that will answer this question.