1998 Toyota Avensis goes to limp mode after reaching normal temperature

This avensis hatch has the last of the 7AFE engines on an "S" reg B4 the upgrade to the VVTI engines on "T" regs. Whether this is relevant I dont know. Back in Sept07, while driving on holiday the engine started stuttering if I exceeded 2.8K revs in ANY gear, in 5th gear this has restricted me to just under 60 mph for motorway driving. I have lived with this problem till now because 99% of my driving is urban and really is more of an inconvenience than anything else. Over the last couple of years I have tried to get it resolved BUT WITH NO SUCCESS. - WHAT I HAVE FOUND OUT- From a cold start, ie mornings, it runs perfectly, I can accellerate to any revs in any gear and if I can get to the motorway quick enough I can drive over 100mph (yeh I know its wrong)- NOW - when the engine gets to normal temp, maybe 6-8mins, LIMP MODE kicks in on the fuel supply and restricts me to 2.8K revs IN ANY GEAR. For motorway driving that means just under 60, for urban driving it means its a bit of a bugger when pulling away and getting up to 30 to 40 cos I have to change gear so quickly b4 revs reach 2.8 otherwise it stutters. Now driving under 2.8k revs in ANY gear and it is PERFECT, you would not know anything was wrong at all. So I summise this is some sort of heat related problem unless told otherwise, that is being detected by the ECU thus causing the Limp Mode to kick in.
WHAT I HAVE DONE SO FAR WITHOUT HAVING ANY SUCCESS. - 3 (THREE) different diagnostic tests, 2 by at-your-home services and 1 by a top of the range workshop that specialises in racing/rally cars and other high spec cars, plus offers ECU problem solving, this place had top of the range equipment. All 3 detected the problem but could not locate the cause, with all 3 the fuel supply registered flat-lining when the revs reached 2.8k, with all 3 the engine ran/behaved absolutely perfect below 2.8k revs. All the sensors checked out perfectly, and the hi-tec diag boys took it out on the road 5 times in 3 hours (thankfully and to their credit they were extremely kind and only charged me 1 hours labour - they were more determined to try and solve the problem than take my money !!), but alas no luck. Since then I have myself gone to a local Jap car breakers yard and swapped and tried another ECU, HT leads, the sensors for cam, water heat, exhaust gas mix, although all these were thoroughly tested and checked by the hi-tec diag boys and all came up with a green tick. To summarise, something is being detected as wrong by the ECU when the engine reaches normal temp, and thus instructs the fuel supply to switch into limp mode when I reach 2.8k revs in any gear. THERE YOU HAVE IT SHERLOCK........I HAVE GIVEN UP HOPE FIXING IT......I'M STUCK WITH IT AND CANT AFFORD TO GET ANOTHER CAR.
Apart from this site which I am trying for the first time, I found the Toyota users site no help at all and I might try Justanswer again after an unsuccessful go about a year ago. GOOD LUCK AND THANKS TO ANYONE WHO IS KIND ENOUGH TO OFFER ANY HELP....WHOEVER DOES CRACK THIS PROBLEM WILL GET A CHQ FROM ME.