I don't think you need an RPM sensor... I think you need a position sensor for the pan & tilt. i.e. a [u]servo[/u].
An RPM counter could be a round-about way of making a servo motor, but you should probably just get a real servo motor.
I have been able to control the speed and direction of the motor by PWM. But as the RPM of the motor decreases, the stall torque of the motor decreases as well.
As mark says, that's pretty-much the physics of a DC motor. The only input-control you have is voltage (via PWM or whatever). Current is then determined by the applied voltage and the load on the motor... i.e. if you put a load on the motor to stall it, current will increase, and if your power supply can't supply the "needed" current, the voltage will drop.
With the proper servo-feedback, the required voltage can be supplied for the proper torque & speed to position the motor at any moment in time.