"For addressing the 128K RAM, I have used Pin-1,(For A16 pin).Inputs for decoder are pin-2 and pin-3 as A17 & A18."

Of which port ?
why A17 AND A18.

"Some times, my NVRAM also gets automatically disabled from read or write operations."

two possibilities
1) you are still selectiong the decoder.
2) you are using capacitor reset.

It all seems that you are not handling the 'switch' between decoder and memory the right way.

Which compiler are you using?

Erik