Engine “surge” when cruising

Hi, my car is experiencing a “surge” (not sure what word to use here) between 30-60 mph. The revs will float between 1,600-1,800/2,000-2,200 while maintaining the same speed. You can feel the car jerking back and forward.

I had a gearbox replaced a year ago at 96,000, with a second hand unit. A transmission fluid change at around 101,000.

Without diving into looking into whether the torque converter is at fault to save money, are there any other causes for this?

Thanks