Problem with auto gearbox - or gremlin ?

[ALLCOCKSU]

I have a low mileage (just under 89K) 2002 Yaris Verso with an auto gearbox.

It's been fairly trouble free since purchase in 2016 - serviced regularly and MOT advisories attended to promptly (it's never had an MOT failure). 

In  mid-October I was returning home when the gearbox developed a fault which in a manual car I would have described as clutch slip : audible 'hunting' through the gears, wildly swinging speedo, but engine revs remained fairly constant and consistent with the accelerator pedal position. This was at about 50-55 mph but the problem persisted as I neared home and only disappeared when I turned into our local road and speed had dropped to about 20 mph.

I only drove the car once after that, to take it to our local garage. At 20-30 mph on a cold engine and box there was no sign of slippage, but the drive to the garage was under 10 minutes compared with the 45 minutes drive when the trouble first occurred. Local garage checked it over and performed a standard gearbox service (whatever that is) and confirmed the problem was still there. 

Car has now been recovered to a transmission specialist and has been with them for a week.  Staff say that they have driven the car twice so far (going out for a longer motorway drive today) and also say they have scanned engine and gearbox, but can't find any explanation for the problem, nor any sign of the problem even at motorway speeds.

Has anyone had any experience of this or a similar problem, or any advice to offer ? I obviously don't want to pay a hefty bill unless sure that they have fixed it, but neither do I want to believe that the problem has sorted itself out without human intervention The next time it happens might be on a  long-distance journey, or when my partner's car needs attention - we have so far shared his car for 7 weeks now, but neither of us would claim that's sustainable in the long run.

[Benzowner]

When was the transmission fluid last changed. I have had 3 cars with CVT boxes which more or less what Toyota use and always had the transmission fluid changed at 40k miles. It could well be the fluid is too old and overheats rapidly which will then affect the viscosity and cause all sorts of issues. You may need to look for the filters that may also need to be changed.