Dave A8

I have 3,0 turbo engine (A8). High mileage but well looked after. The problem is I don't use anywhere near enough but can't bear to part with it. It has developed a fault that my mechanic can't find! If it is in traffic for some time or when I use it for a couple of hours it seems to go into a safe mode and loses much of its power. At the same time the fans have started kicking in almost from starting the engine - I think they must go off because I don't hear them all the time. Both are expensive to even go looking for. it does not happen every time and turn the car off and restart it, normally, goes back to normal. If it is not either of those I can ignore them for a while and look for a soloution. Trouble is, I am loosing confidence and don't want to take it for the longer drives. Can anyone help? Could the 2 problems even be linked? BTW temperature stays at 90 all the time - just where it should be. Thanks everybody.