Oscillation on Pin 13, delay in code?

So I've been using Arduino for a few years as part of a robotics-based thesis, which I recently defended. I decided during post-thesis work to switch to a headerless Leonardo for the sake of reducing the bird's nest of wiring, and it seemed to work for that purpose. The problem is that whenever I turn the power on to it, it will send signals to pin 13 that oscillate the servo I had connected for about 10 seconds, after which it will run the code normally with no issue. I circumvented part of the issue by swapping the servo to another pin, but the code delay is still there and is incredibly annoying. Can anyone advise as to how to diagnose and address the problem?

The problem is that whenever I turn the power on to it, it will send signals to pin 13 that oscillate the servo

If you have the bootloader installed this will happen. (for an UNO anyway)
Try using ICSP programming.

Edit: Or use a different pin.