Need Help With Mach3 spindle index problem using proximity sensor

Mach3 spindle index problem using proximity sensor problem appears to be due to noise.

Machine Description
I am setting up a lathe on Mach3Turn. Using Mitsubishi Servo to drive axis and LG inverter for 5HP spindle motor running on Dell Laptop D600. I have installed a proximity sensor for spindle RPM with opto isolator following circut explained on Mach3 forum at PWM output for spindle
http://cncrouter.ru/images/lathe.gif

Problem
My spindle RPM gets disturbed as I switch on my Servo Drives. I tested it by removing Step/Dir wires from my break-out board.
Initial run shows RPM much stable with variation of 1-2 RPM running at 600 but as I switch-on my Servo drives MEL-Servo-J2S my spindle RPM starts jumping Mach shows it 598,670,712,1040,2482 and many such figure. As I understand it is noise but despite of many efforts it is not being resolved,
I tried shielding wires
grounded the machine
Installed Schmitt Trigger using 555 (but results more worse showing RPM 126306)
Moved Breakout board to a separate box away from servo drives

What else should I do to get correct RPM please help!

Thanks