Which pin is giving you your % reading ?
This sounds like an intake air leak, as it does so when leaner and warm.
Check the hose/s to and from the idle valve...they split.
Are you unplugging the O2 under the passengers side carpet ?
Are you sure you put the distributor cap on square and did not loose the center carbon contact.?
The engines on these will hunt at idle if the % is not set as close to 50% as possible.
The rear 'two pin' sensor on the intake side of the head is the CTS....warm should read 380 ohms from either pin to ground.
Cold is 2000 ohms or more to ground.
Is the missfire regular or random?
Which cylinder if any ?
Enquiring minds want to know.
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