The following suggestion can be analysed.

noise spike gives more interrupt to serial port hence more data is read and displayed

are u using smps, DC/DC power supply for microcontroller board. if so there will be noise spike appearing at dc output/vcc for micro. u can check using oscilloscope.

PC mains power chord should have three pin, earth should be proper meaning potential between neutral and earth should be less than 7 or 4V.

u can use 0.1uF/0.01uF Y1 or Y2 capacitor between micro supply negative terminal and pc side negative terminal.

optocoupler will not solve the problem.