Yes, I am using software serial
how do I solve this problem ?
a) Use the hardware serial port on an UNO and give up on Serial I/O to the PC
b) Use a Leonardo which can talk to the PC via USB(Serial) leaving the hardware serial (Serial1) free.
c) Use an Arduino MEGA which has several hardware serial ports
d) Use a Timer/Counter to produce servo outputs in hardware without needing interrupts.
e) Use a PWM shield to control servos with hardware.