About Nikon D750 Camera Body problem

When I shots in manual mode shutter speed changes after the shot is done such as at 800 iso if the shutter speed is 1/50 5.6 immediate after the shot done shutter speed changes to 1/40 or 1/80 no change in aparture value (exposure compensation value also changes when it is saved at 0.0 showing in histogram). When taking shots on S mode no change in shutter speed.
I don't know what is the problem in which part ?
Is there any solution or go to the nikon's service centre....but here in kolkata service centre some persons have bitter experiences.......I don't know now they start using modern machineries.
I have a good experience at Canon service centre (1day I gone with my friend) they take his canon lens open it with machine....so on.....we r taking tea/coffee & watching the procedures.....
Is there this kind of modified machineries at Nikon's service centre in kolkata, India.

Welcome to the forum. You may want to see if "auto ISO" is turned on in the menu setting. If it is, this is the most likely reason your shutterspeed changes. If the auto ISO is set at a maximum certain threshold, the camera won't go above that setting so your shutterspeed drops to get a proper exposure.

When you're in "S" mode then the shutterspeed speed stays the same.

Welcome aboard. Enjoy the ride.
We look forward to seeing more posts and samples of your work.

Hopefully the suggestion above about auto-Iso solves your issue.

I suspect that you have inadvertantly turned the "Bracketing" mode on. Does the exposure change as well between the shots you mentioned? Check the shots done in S mode, do 3 or 4 of the same scene and see if the exposure is changing between shots. If it is, you have touched the BKT button while turning the main control wheel. See manual to disable it if it was turned on.

Hope this helps.