When trying to maintain a speed between 35 & 50 and the rpms are 1200-1500 the engine surges. Even on cruise control, you can watch the rpm guage swing by more than 100. It's not the transmission or torque converter as the trans gear and lock-up status don't change while the surging occurs. The only thing that makes a difference is a fuel additive, which reduces the surging considerably. It only surges at lower rpms when very little power is needed to maintain speed.
Surging at low power & low rpm on 2005 Acura TL
If a fuel additive makes a difference, then I would scan ( or have a trained tech do it. I don't mean a parts store quickie code check which I find causes as many problems as they repair ) the engine for codes and really check the Fuel Trim numbers ( both Long term and Short term ) at the speeds that concern you. You could have a lean mixture problem that is causing the surging. It could be a sensor or a vacuum leak etc. Do you ever get a Check Engine Light coming on?