elegance est auto 2004. Juddering and cutting out, intermittent

Howdie reader :-)

Having intermittent problem of juddering around 3rd gear - sometimes speeds up temporarily and often just rights itself. Also problems with starting and difficulty accelerating after slowing down. A good rev seems to kick it back in. Has full service history, 104k miles and passed mot in Feb. We are about to sell this car but don't want hand it over with this problem. It's due a service now. Was hoping someone could point us in the direction of where to look during the service to sort this out at the same time. It started last year, comes and goes, is generally better after fuel is added. We also use Ametech products to clean/repair in the fuel and oil.

Happy to give more info. if needed.

Cheers