Mystery gearbox fault

[Four Oaks Bookworm]

Bit of a mystery this one, our gearbox (2.5l diesel) is hunting around for a gear and not seeming to find the correct one, but only under a very specific set of circumstances:
- when the engine is warm (never does it first thing, but gets worse after e.g 1 hr of running)
- between 1500 and 2000 rpm
- (mostly) between 4th and 'overdrive', only very occasionally between 3 and 4
- around the 40-50mph mark
- when pressing lightly on the throttle to e.g go up a slight incline on a major road (not when 'hoofing it' up a steep gradient)

We've had the gearbox oil changed, and this made a difference to the change itself (much smoother), as well as removing the slight jerk when selecting D or R. But the above problem has persisted.

Any ideas folks?
thanks

[Ian]

Here's something similar that I found in the "Clinic" on the main BF website:

Q: I’m having problems with my gearbox. It doesn’t always seem to know which gear it should be in, especially at greater speeds. So, starting from cold, it changes gear OK, sequentially, up through the box without any particular clunkiness, and finally moves into top gear. However, if, say, I was then cruising along at 60mph in top gear and driving circumstances in front of me required me to back off of the throttle, with a resultant loss of, say, 15mph, reapplying the throttle would cause the gearbox to appear to be uncertain about which gear it should get back into. What’s the likely cause? And will I need to take it to a tranmission specialist? (Andy from Battersea)

A: Identifying and fixing auto-box problems can be time consuming and expensive. So before you go down that route try the following. Not all gearbox problems are actually gearbox problems at all. Lots of things affect when the gearbox changes, including the fuel filter, banjo filter (top of the fuel pump), throttle position sensor, and turbo pipework leaks. All these can fool the gearbox into conflict.

[Four Oaks Bookworm]

Update on this post, since the problem has still not really been identified (even by a 'Bongo specialist' garage)

Gearbox oil has been changed (now twice), with 'antivirus' additive
Fuel filter changed

It's such a specific problem and we're at a total loss. Just to recap
- Only when warm (after at least 45-60 mins of running), never cold
- Only 1500-ish revs (this is usually 3-4 at around 35mph, or overdrive above 40)
- Only with light throttle
- No hold light flashing
- No problem engaging D or R initially, and changes are smooth
- No problem accelerating/power, so I doubt it's the turbo

Hope someone can help, we're already £700 down and still haven't got to the bottom of the issue. TIA!

[Northern Bongolow]

To be honest it sounds good to me, revs and performance sounds about right and as expected.
It's possible the throttle position sensor is occasionally not getting the message, what age is the bongo. Early ones had tps on top of pump, later ones had it on pedal.
The manual shows you how to test and adjust, this can sharpen up the gear changing .

[Ian]

There's a slightly annoying video on testing the TPS on a Ford Ranger (same thing) at https://www.youtube.com/watch?v=zDNrOQm42Ng

[Allans]

If no hold light flashing, it's generally outside scope of ecu. But the airflow meter can be faulty and not produce a code. Just guessing though. Can't be sure.