Stuttering

I just recently bought this car. When I got the car it was jerking while driving. Upon checking all the normal things it was found that the previous owner had put in the incorrect spark plugs, oil filter, oil and fuel.

I have subsequently changed the spark plugs, oil filter, oil and I am ensuring that the correct fuel is put into the vehicle. This seemed to have improved some of the performance.

I then subsequently took it into the workshop where they advised that the fuel pump needed to be replaced. The fuel pump was replaced and fuel tank removed and cleaned out.

I have taken the car back to the workshop and had them run a diagnostics check on the vehicle and no error codes were received.

The car is currently stuttering when pulling away. It has limited power, idling is a little erratic and sometimes it will not pass 4000 rpm's in first to third gear. It appears to drive well at freeway speeds, but when you drive between 60 and 80 in fifth gear it tends to jerk.

What could be causing this problem as the workshop cannot seem to find the problem?