Very difficult to diagnose autobox faults without driving it, but, if what your saying happens over 30/40mph then is sounds fine.

All autos now have torque converter lockup, this is where the turbine and impellor become equal in speed. Something that the older autos never did, hence poor fuel economy in automatics as you always had an over drive scenario of engine to box.

With lockup the engine speed becomes 1:1, what the crank delivers is what the box receives, usually a drop of about 500-1000rpm

n.b. lockup only occurs when you back off the throttle i.e. very light cruise, as soon as you accell again it instantly releases.