Stutters loses power and stalls

i bought my first merc thought it was great when test drove. took it out for a good run and it stutters after a while, loses power which is extremly irratating especially in rush hour at roundabouts, had it checked over got told it needed a new coil rack so had a new one installed, got told the spark plugs were fine they had only recently been changed, started driving ok was good for longer than normal and then it started all over again, stutters loss of power and stalls (bearing in mind this is an automatic i was told they never stall but this does) can anyone give advice on what may be wrong now