Engine "surge" during constant speed

[John Lenin]

2009 Yaris 2 Dr. H/B automatic, 41,000kms (25,477 mi)

Have noticed that while driving at a constant speed at approx. 60-65kph (30-40mph) on a level road or slight downgrade with tranny in the "D" drive mode, that my tach will show revs approx. 1200-1500 RPM and then suddenly drop to 1000 RPM or slightly less, making the car "surge" like under engine-braking. I don't feel like I've backed off the throttle but revs climb back up again to original level once I do apply a bit more toe to the gas pedal. This happens intermittently and mostly if I am travelling along a long stretch of level road (or slight downgrade) at as I said, a constant speed.

Car is a recently purchased Lease Return to replace my previous 2008 Yaris H/B 5-spd which I could no longer drive on account of a broken left leg ... which I was very happy with BTW ...

All comments welcome and appreciated!