John, you have my sympathy.
I used to run a standard 1991 Plus 8 3.9 efi with no ecu upgrades and no cats. Sometimes the car would, for no reason run at 2000 to 2500 rpm either on start up or refuse to drop below these figures when fully warmed up.
Although this behaviour only lasted for a few minutes at most each time, it could be highly embarrassing.
Of course as it was an intermittent fault it never did this when with the dealer
Unfortunately I have no solutions for you but feel your frustration. Good luck with finding some solution.
