Bikram,

The reason that it works sometimes and not always, is that your switch is not de-bounced.

Try using a 10uF capacitor and 10 K resistor combincation in the following manner.

Connect cap + to Vcc. Cap - to RESET.
Resistor to RESET and GND

I have used it on 100s of circuits and never found any problem.

Good Luck.

IA