Dear Braintrust

Ive got a Gowesty 2.3 engine in a 91 Westy that I did a top end refresh with rings over the last year. It s my first water boxer rebuild and Im happy to say it is running.
After starting it idles well and revs up normally. A few minuets later it starts to stumble on revving and will die with a hard throttle push.
While driving when it starts to stumble, turning  ignition off and on will cure it for about a minute or two.
Probably kicking ECU back into open loop for bit?

I have not found any vacuums leaks. The timing is set at 35 degrees btdc.

I found that disconecting the o2 sensor wire solves this problem. Engine runs good. Revs normally and climbs hills well
It is running rich as expected. 

I swapped out the o2 sensor that came with the used  SS exhaust system with one the seller sent with it that looked new. No change. Same problem.
Ive got one of the Bosch 13957 sensor that Gowesty recommends coming from ebay.

With engine running and o2 sensor connected, Ive got 0.92 v  sensor to ground. 
Running and O2 disconnected   .91v on sensor and .44v on harness to ECU
Sensor wire is not shorted to ground. o2 heater has 12v

I have
1 TB switch set
2 Blue temp sensor is good. Resistance and Temp same as Bentley graph. Also used a 300 ohm resistor to simulate hot engine.
3.Fuel pressure 29psi vacc hose off. 36psi hose on.
4. Swapped out with good AFM no change
5. Swapped out with good ECU no change
6. Swapped Distributor no change
7. ECU wiring harness  pin 10 gray to ISU and Temp Sensor good
     pin18 green to Hall and ISV good
     pin 6 brown/white to hall, TB switch, Temp sensor, AFM, good
     pin 2 to O2 sensor good
     pin 19 to ground good
8 Vacuum 15 “ at idle

Whew: I think thats what Ive done. Other than trying the new o2 sensor from ebay. 
What am I missing?
Thanks to all of you.
Bob
 
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