Auto Box Problem - 2.8 Ghia X

[dipsomaniac]

Engine revs to around 5/6000 rpm before auto box changes gear (all gears). It only happens intermittently and after I have done around 100 continuous miles. When cooled, seems ok again. No faults reporting on the auto box, EVAP fault (I think Ford damaged it when fitting a new water pump) on the engine. Auto box specialist took car for a 10 miles test drive but the it drove perfectly - typical. Gearbox was serviced 2 months ago. Could it be a sticking solenoid when the box gets hot, if so, can this be identified electronically when the fault is occurring, and be repaired without removing the box? Any help would be appreciated.