audi s6 -w reg - gear ratio fault

my car seems to judder violently when i get past 30mph on level ground, on hills its worst. when it happens there is a bold red line that greys out the dash that shows the gear selected. when it greys out, it drives ok, but very slow. when it grey out and i stop, starting off again is very sluggish " more like in 2nd Gear". i took it to a garage and diagnostics was done. gear ratio fault seems to be coming up everytime. the error codes have been erased several times but the problem is still there. Any help will be appreciated. i am pulling my hair out . its very embarassing when i am on a fast moving road. i have to stop , switch the engine off, which clears the fault, but as soon as i get to 30mph or go up a hill the fault reappears. thanks