thunderbird super coupe auto (aod) third gear seems to have turned into neutral !

hi i live in the uk and was driving my tbird (3.8l supercharged v6) doing about 70 mph in the overdrive gear (auto) and all of a sudden the gearchange to 3rd was a bit of a jolt and i lost 3rd , its now neutral. the car still drives fine in 1st and into 2nd but when it goeas for 3rd there is nothing there, it just revs. i have tried in the normal drive and still no third. reverse etc everything else works but im stumped as im here in the uk. i have not lost fluid in he box as its still topped up correctly.
i have heard of a tv cable to check but have not checked yet. am i in need of a new gearbox???