In Manual Mode the shutter speed (or the aperture setting) should never change from what you have set it. You should also have the "Exposure meter" showing on the top LCD.
Since setting the EV on the camera has no effect (in M Mode) then all I can think is that your selector switch may have an intermittent fault. I would "exercise" the selector switch a few times and see if that helps.
Regards, Clive Liddell Pietermaritzburg South Africa