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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.