shuddering AU

[Bait_Fish]

hey guys
my poor baby girl is having a few problems. when i start her she fires to life but then the revs drop n she shudders a bit, almost feels like she is going to stahl, then comes back to normal, untill i put her in the gear, n the same thing happens again! i was at the lights the other day n i turned the ac on it did the shake! shes an auto forte S3 with 83000kms on the clock...
so would this be the fuel pump? or the filter or something totally different, oh n fuel eco. has gone out the door about 400kms out of a tank on the highway, before i was getting around 600kms! help please!!! she did it, as i was putting her into gear to reverse out she stahled! just died! but started up n off i went!! help really needed!!