It's cause your engine's not supposed to pull a car at that low of an RPM.... if possible, never let your RPMs drop below 2K while actually driving your car... at idle, you can let it sit at ±750 RPM, but your engine will NOT like you if you drive any less than 1.5K RPMs, and it will do fine if you keep it above 2K....
Anyways, you're making like 20 hp and who knows how little ft/lbs of torque at those RPMs.