I have tried replacing the relay with multimeter and i was able to see the high pulse for about 2-3 seconds (3.3 volts) at startup.
So i guess its the sensor board which is abnormal here, and not the relay. Because its the sensor who is giving high pulse at startup irrespective of relay.
Also, before using 1000uf capacitor, I had already tried 100uf and it didn't make any change at all.
And yes, PIR is also behaving exactly similar to rf receiver board at startup.
I have also tried with different cellphone smps power supply which is more stable.