Bighorn Transmission Flashing light

I have had a 1994 2.8T Bighorn Lotus for about 2 years. It has an automatic transmission with overdrive (and a lock-up clutch by the sounds of it). I
believe the transmission was serviced just before I bought it.
A few days ago the 'Overdrive Off' light started flashing. It doesn't flash when activated manually with the overdrive off button. Generally the flashing only happens when braking although it also does it sometimes when idling. Speeding the engine up slightly off idle stops the flashing. I'm guessing therefore it's an oil level/flow related issue.
I've never lost any oil though and the level appears O.K it also seems nice and red and clean, so I'm a little perplexed. Unfortunately it's Xmas time and I plan to head off on holiday in this vehicle in about 10 days time. Hopefully I'll be able to get it into someone to get it serviced before then but I'd like to know what's causing the flashing.
Any suggestions?
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