Loss of power

the car will be running fine, and then all of a sudden when revving, accelerating' there seems to be nothing there. I then feel like I'm having to thrash the lower gears although it wont be going over 20/30 mph!!! if its safe to do so, ill pull over turn engine off then restart it. if it will rev up nicely and normally ill then go to drive it again, and it will drive fine again maybe for the rest of the day sometimes, but its only a matter of time before it does it again!! this car maybe old to some people, but ive not had it long, it only has 92000 miles and full service history with it!!! please can anyone help me, as I'm worried when driving it with my to young children in the car!!! I brought a fault finder today and it said there is no faults coming up on it???