Your post said power seemed normal at higher R.P.M.s and that the trans is starting off in first gear.
An often overlooked cause for poor take off is the torque converter,on take off the stator turns and the converter acts as a torque converter acts as a torque multiplier,as engine speed and input shaft speeds equalize the stator locks up on a one way roller and the converter acts as a hydraulic coupling.
A bad torque converter will cause poor take off acceleration but will drive normally .
Have a good trans. tech. road test the car!