Reocuuring P2096 Error code

[RobF]

Help please. I am out of ideas on how to stop an occasional reoccurring CEL with a P2096 ( Post catalyst fuel trim system too lean bank 1) error code on my 2003 2.7 Boxster, FSH 72K miles.

I have owned the car nearly two years. The car has always been well maintained and is running beautifully throughout this period. Everything is working exactly as it should be.
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The CEL/ P2096 code pops up every 150-200 miles or so, which is easy enough to clear using my Foxwell OBD2 tool, but is annoying me.

So I have on 2 occasions put it in to a highly respected local Porsche Indy, to investigate and fix.

On the first occasion, about a year and 1500 miles ago, the 4 O2 sensors were replaced with new Bosch sensors. This did not stop the occasional CEL problem.

So last week the Indy spent a couple of days checking every possible other cause, including:
- live diagnostics and live data -pass, no strange readings
- fuel rails checked and pressures checked - pass
- manometer check on vacuum system - pass,
- full visual check of exhaust system, cold and hot - pass
- spark plugs removed and inspected, no evidence of lean running on either bank, nor any injector issues
- O2 sensor wiring checks - pass; and plug connectors checked cleaned
- air filter check -pass
- Engine bay pipework inspection - pass
- smoke tests of intake system, vacuum system, AOS, Exhaust system warm & cold - pass
- Fuel cap and oil cap seals, fuel vent check - pass
- MAF sensor inspected - pass, and cleaned

We assume the CATs are ok as there are no emissions issues.

To rub salt in the would the CEL light came on again on my drive home from the Indy.

Am loathe to give up and accept that this P2096 code is may be an unsolvable "ghost" or random "cable-earth" cause, so thought I would ask this forum, anybody else had this issue and if so what was your fix please?

Rob (in South Wales)