355 Of aftermarket exhausts, misfires and spark plug heat ranges

Late '95 build, 2.7 F355 with air bags.

The car runs Tubi headers and a Capristo muffler. All 4 cats are gone. NGK CR8EIX sparks plugs.
Out of the blue the car started misfiring on one cylinder, after some repeated full load accelerations to 8000 rpm. Changing to the next gear at that pace always causes backfiring, as if the fuel cut when closing the throttle was not effective and if extra fuel was ignited in the exhaust.
The misfire issue seems fixed so far after cleaning all connectors in the harnesses and going back to the PMR7A plugs the car came with, but I still get backfires as I close the throttle abruptly when changing the gears up. It seems that closing the throttle slowly rather than abruptly when changing gears up massages down the backfire but it is still there.
Injectors were cleaned and flow tested. FPRs have been ruled out. Disconnecting the battery and letting the ECUs relearn makes no difference.

At this stage I suspect that the higher flowing exhaust set up makes for a fresher and increased air flow that the colder 8 range plugs, despite being the factory recommendation for a 2.7, and the factory programming of the fuel cut that may not be sharp enough, cannot cope with.

What am I missing?

PS : this car is amazing. It makes everything else feel dull.

 

If the fuel cut was not effective on either side I would expect repeated backfires rather than only just one.
Also with a faulty TPS I would expect the corresponding ECU to not know when it is supposed to trigger idle parameters and the engine to struggle to shoot for idle speed through the corresponding ISV. Idle quality is not on par with a modern engine but I have read on here that was a feature of 2.7 F355s.

What should either TPS return for resistor values?

 

Just found this posted by @Qavion a while ago. Thanks Ian. I will check what both TPSs return.

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Well, here are the values in kohm measured (throttles Closed / Open).
Both TPS seem to be the originals at ~80k km / 48k miles.

LH TPS (Bank 5-8) :
1-2 : 1.72 / 1.72
2-3 : 2.37 / 1.03
3-1 : 0.96 / 2.25

RH TPS (Bank 1-4) :
1-2 : 1.87 / 1.87
2-3 : 0.99 / 2.39
3-1 : 2.47 / 1.06

Not really conclusive.