To clarify - Is this a cordless or corded drill?
A pot is short for a potentiometer which in essence is a variable resistor. Generally though, these have to be wired into a circuit of some description as they are not capable of dealing with high current throughput. If you're not sure how to do this I'd advise to steer clear of DIY electric circuits, especially if this is AC.
The problem is that universal motors are very difficult to get running at a constant speed under load, unless the load is phenomenally stable (like a fan). AC induction motors are intrinsically self-regulating in terms of speed. However, with universal motors, torque is (roughly) inversely related to speed for a given voltage. This means that the motor will slow down to meet the applied force/torque. By reducing the voltage, you're reducing the speed but also limiting the maximum torque the drill can apply and this is exacerbating your low-speed instability.
There are two ways to control a universal motor speed - voltage and pulse width modulation (PWM). Unfortunately, either method will not compensate for changing loads without some form of speed feedback (eg a shaft encoder). Reducing the voltage through an external means will not improve the low-speed running stability.
The best option would be a gearbox or induction motor if you're keen on getting a more stable speed.
