94 LT1 rough idle, cuts out

This 25th TA only has 22K mi on it. It started to cut out and die a few months ago, and has been unreliable since. I extracted the trouble codes.
There were a few odballs that didn't come back after resetting, but DTC 36 came back. I followed the diagnostics in the factory manuals and concluded that the hi res sensor inside the distributor was bad. I replaced it and now it idles a little rough all the time and gets worse as it warms up, to the point where it quits. There are no trouble codes in the t computer, but it is running open loop. What causes the computer to switch between open loop and closed loop? What else should I check?
TIA, Paul
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