Help lost overdrive on 1992 4ws4wd GTO auto

Hi all
Was coming back from Wales and had to turn sharply doing about 40 halfway round roundabout and when I came out the other side the engine lost power. stopped and started and ok (still with OD button engaged) and it slowly lost power as put foot down on accelerator. Dont know why I tried with OD off but I could then drive the final 35 miles home in drive (went around 55MPH).
I imagine OD has a drive cog but if this went the gearbox would be gone? I know originally 7 years ago my guy replaced the"TCU" at back of dash when I lost everything , could this be reason again ?
If so anyone got one for last of MK1 as above or maybe knows what is happening, cant lose this one!!!

I have a: (GTO, 3000GT, STEALTH / SL, MR, VR4, TT)
It has the following issue: (be sure to describe it as best as you can. Remember to also tell us what you have tried so far)

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