If these are 360-degree servos common with robotics projects (for drive wheels), then there's probably not much you can do. Those types of servos have been modified where they are more akin to reversing motors with a gear-reduction drive train. They don't have positional accuracy needed for airplanes.
If they are "normal" servos but have a full 360 travel that has a working potentiometer for positional accuracy, you will have to reduce your output PWM to a very narrow band to make them work. Most servos take a 1000-2000us PWM pulse that delivers the full range of motion (with 1500us being the center position). If that range equates to 360 degrees, you'll only be using a fraction of that range (possibly 1400-1600us. You may not have enough resolution for effective control. Most airplane servos only have 90-120 degrees of travel.