2005 Toyota Avensis no accelaration after 8 miles of drivin

After driving on the motorway for 8 miles the car will no longer accelerate properly. Along the flat and dowhill it will accelrate slowly but on any incline speed is lost. The only way to accelerate is to shift down to a lower gear until the revs are above 3,300 at which point normal acceleration resumes. After a further 15 miles the fault clears and the car drives normally for the rest of the day even if parked for a few hours. My main toyota agent has tested the car and can provide no resolution to this problem since there are no fault codes present even

when the fault is present and it is being driven. Their realtime diagnostics showed nothing abnormal even though the car would not accelerate.