PeterH:
Seems as if you may be getting corruption on the connection. Does the library you are using support CRC? The RF chip does. If it is supported, enable it. I don't know anything about that library, but if it's any good it ought to enable you to configure the CRC checks, and also timeout and retransmission options. If so, the examples ought to demonstrate how to use these features. If you don't find that information, consider adopting ManiacBug's RF24 library instead - it is just as easy to use, works well, and comes with plenty of examples.
Thanks again for your kind response. I shall try it with RF24 library. Visited the Maniacbug blog, it is nice. But the point at which I am stuck is the functions of the RF24 library. Right now I'm googling for a in-detail description of those RF24 functions. In the meanwhile my DHT11 is burnt due to uncontrolled power supply from wall adapter... 
I used a cheap 12V 1A wall adapter & it destroyed both the ATmega328 & DHT but FTDI is OK. Yesterday I replaced the Mega328. Going to test with LDR & Thermistor now.