HELP: Sirion X 2000 F-speed - Sudden intermittent problem with auto gearbox

Forum: 

Sirion 1.3 automatic F-speed X reg, year 2000. 100,005 miles.
Here's what happened:
After driving for about 1 hour, near home. No strange noises at any time.
On a slight UPhill, idling up in first gear, Manual first selected, in traffic.
Suddenly there's a "boink" at the front and car seems to drop a bit at the front. Stops moving. Applying power a bit feels like car in on handbrake.
NO caution nor warnings on dash. Neutral selected, handbrake put on, Hazards on. Back to drive, same symptoms.
Park selected, engine off. Engine on, no warnings. Drive selected, no symptoms, drive car around corner onto side road and park. Cannot see any debris nor fluids. Check auto trans fluid, all ok.
U-turn, out of road, back down hill all okay, get into 2nd. On flat section car drops at front, drag occurs again, neutral selected, CAR ROLLS / COASTS fine. Select Drive again, car fine, turn into home road, sharp left turn next with slight UPhill gradient, car goes wrong again.
Engine still on (for brakes), push car home, 100 yards, roll down hill into drive BUT with big bounce over curb to park up.
Breakdown company arrive. Can't find anything wrong with auto-gearbox. Checks driveshafts, all fine. Drives up and down my road at many many speeds, making sure box changes gear, and reverse, no problems. Then checks for mounting problem. Engine appears to move a lot at the top, but not obviously broken mountings. No fault found. Possible mount problem reported on form.
Take car to local trusted garage coincidentally 30 yards from where fault occurred at top of slight hill. So about 1/2 mile total. Downhill, uphill, gear changes, 1,2,3,4 and reverse used.
They can find NO fault.
Unknw
Anyone had any of these events? Can this be debris in auto fluid problem? Can this be filter issue? Debris moving in auto box? I am told this is a completely mechanical auto box.....
Any ideas?
Hot fault but not when cold?
Most grateful to anyone reading this far down.

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When was the autobox fluid and Filter last changed?

If this is like the YRV Turbo and F-Speed auto box then there is a service interval of every 3 years and as well as the auto box fluid to be changed,( it's misleading looking at the auto box dipstick as the cherry red colour is no indication of the actual fluid inside the auto box) there is also a auto box filter. Unfortunately this is not mentioned in the Daihatsu Service booklet you get with the car, and was only discovered a few years back by looking in the back of the owners manual. If you asked a Daihatsu Garage about it, they wouldn't know. Although the Auto box filter can be bought from them for around £50 if they look it up on the parts CD. Had mine done over a year ago, the dipstick when pulled out would show a nice Cherry red, but when the oil actually came out of mine it was more of a black colour, and the filter that came out had seen better days. Both changed for piece of mind.

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'12' Plate Mazda MX-5 2.0 litre NC 3.5 Venture Roadster 160BHP
'15' Plate SEAT LEON 184 FR TDI. 240BHP/500nm Torque.
Past: '53' Plate Yellow YRV Turbo Approx. 150BHP