Just the other day, the gearbox decided it would work propely and select fourth gear instead of revving its head of in third gear banking up the traffic

I wonder how long this will last! this car has many suprises in store each time my father and I drive it. The common question that we both ask each other after one of us has driven it is "what the hell did it do this time?" and we answer accordingly, usually its "oh the BLOODY THING WONT SELECT OVERDRIVE!"

Oh well, ya cant have everything!

Check the OD lockout on the transmission. it's a mechanical sensor that prevents the OD from engaging in any gear but 4th. the litle rod will wear down until it is intermittent, and wil do strange things like engage on hot days (metal expansion) than on cold days (metal contraction). It will not work for no reason at all, then work just fine. It's easy to pull, and you can test the system by pressing the rod in with the car off, but ignition on (not running). Or, if yours has a washer on it, remove the washer and put a little sealer on the threads, and put it back in place. the removed washer will give the sensor enough depth to work properly.

Probably the OD relay, but the simple control circuit is easy to diagnose.

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OD operation involves 5 components and the wiring that connects them...

1) Fuse 11 (200) or 12 (700/900) supplies voltage to items #2, #3, #4, and #5

2) The Shifter button/switch turns the OD relay off and on

3) The OD relay controls the Upshift (not "OD") Arrow light and Solenoid (separate wires)

4) The Upshift Arrow light ON means the relay and OD are OFF

5) The OD solenoid allows OD engagement when ON, (the "default" Relay state at Key On)

If the Shifter switch turns the Arrow light (#4) on and off, then items 1, 2, and 3 are OK, and the problem is most likely the undercar wire to the Solenoid (#5) or the wire connector terminal at the solenoid.

Less likely would be be Solenoid itself.
Also possible (but unlikely) would be the relay controlling the Arrow light (#4) correctly, but failing to activate the Solenoid.
An internal transmission problem is also possible.

Consider this recent reply to a "check the wiring" suggestion:
"Replaced corroded [solenoid] connector and length of wire with bad insulation. Working perfectly now."
He was having "intermitent" OD failures, and was sure it "couldn't be wiring" because that would be "all or nothing".
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Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.