"The signal wire from the receiver is sent to an arduino input. Another arduino output is then connected to the servo signal input. "
An important part of this is digital logic levels.
Many of the receivers out there are 3.3 V logic, so their 'HIGH' isn't a guaranteed high for the 5V arduinos who differentiate at 3 volts. This will lead to many false signal interpretations (you can see if you rig your code to print an alert if the PPM signal length is outside the range of the normal ~1000-2000 uS of servos).
A simple solution is to use a 3.3V uC, but you can also use an analog comparator circuit to condition the receiver's output into a more recognizable 0-5V
- just be careful that your opAmp or comparator can go close to your rails, or provide the comparator/opAmp that much more voltage so that its output is the desired 5V.