Magsi Sahab, as per my knowledge this is issue with tyre balancing. Put proper weight on rims to balance the tyre. If you are using Chinese Alloy Rims then it could be bad rims itself.
Go to some good Tyre balancing shop and try to switch the tyres .i.e. put back tyres in front and front in back. Usually shop owner see which tyres are more smooth (less problem) and recommend you to put them on front wheels.
I'm not sure but you can try visiting shops at Circular Road or backside of Agha Siraj Complex.