"Also some of the functions of key (such as program , stop) get energised even without pressing the key. This happens once in hour."

Such faults that happen after a predetermined time are mostly due to heating of components; and if they are already operating at the limits of thier design then a most sure reason. Re-Check if any of the components are operating hot and ensure adequate cooling for the electronics.

I would also, if possible, look at reducing the clock frequency. And also look out for any new load that comes on the power line when this problem happens.

As to grounding I can understand your dilemma. I have seen many people quite confused as to what to do with the shield. And true to form this is a huge topic by itself. In the absence of specific information I can take a guess:

*You are powering from the mains. And using a double insulated transformer.
*The plug point has a good earth. (The Earth - Neutral potential is less than 1 to 2 Vac.)
*Your DC power supply common is a star point at the the DC regulator "0 Vdc" point.

If the above are true then your Ground is that star point. Connect your shield with as short a link as possible to this point. And leave the other ( far end ) of the shield un-connected.(Some members may not agree here ...!) And dont forget to connect the metal chassis of the equipment ( if it has one )to the earth pin in the mains plug. This is for safety.

Now the million dollar question: Are you to link the star point to the Plug point earth pin ? I think if you connect then it should be via a earth choke or inductor.

Note : The above is a suggestion. The ideas may or may not be suitable if your end equipment is for medical or mission critical applications.

Raghu