HONDA CITY USERS -- URGENT HELP REQUIRED !!!!

hey bros, i have got my Zero MEtre HONDA city IDSI manual , two weaks ago , MAsha Allah its beuatifull and luxury ! no doubt , but i am encountering a serious problem while high speed drive at HI way /.

the problem is , When i go to Hi Way , KHI-HYD , and the speed is above 100 km/hr , the CIty starts getting shaky , although it remains stable from inside , but she tends to move her path while moving forward , that is , it gets very difficult to keep the car in a straight line , and if i leave the steering for a second , car gets towards extrme left or right .! i have to apply constant corrections to steering , even then i feel , and it practically does happen that , City , doesnt moves forward at high speed , in a stright line and it continously , gives me the feeling of ZigZAg , and if i leave the steering it will never go straight ..!

so WHat is it .? i feel this effect specially when driving against Wind , but even with the wind i get this effect with lesser intensity !

NOw my Alto doesnt do that , and if i maintain a strong grip on her steering , Alto goes straight .! As 100/110 km/hr is very normal speed as comare to big cars , like HOnda City..!

so , What it could be/.??? Remember i m using stock tyres..!! and i thynk it might b narrow stock tyres or allignment issue or suspension or steering.? hOW can i get it solved.? and is this normal with stock tyres ? like its a trade mark City problem.? or its just a new thing.? off course i want to get rid of this , atleast city is suppose to go smooth on motorway , with 120 km/hr..!!

so advices plz.? shld i go to HOnda dealer and claim some warranty? or changing tryes would solve the issue for good.? and if yes then what tyres are suitable.? and i HOpe CNG has no effect on this issue..!!

plz do send m ur advices and experience regrading this..!

i love this CAr and don wana get dissapointed wid dis issue , as its a very basic requirement from a 1300 cc car that she must go stable at 120 km/hr atleast..!

i have just got CNG installed two days Ago..!!