Car vibrations problem?

ok,i've posted this prob before.some people said that the idle were not set right.i've fixed that but the car still vibrates,only slightly better.if i set it any higher fuel consumption will be too high.im using a manual transmission car.whenever the rpm goes below 1000,for example when the car is moving at gear 1 without pressing the gas pedal,the whole car starts to vibrate really really bad till the dashboard,steering wheels,and opened doors make irritating noise.we've been using the car for 12 years,and its using carburetors instead of fuel injectors.it also vibrates badly during cold start.i have a friend who is using the same car model,but it doesnt vibrate as bad as mine.wat could be the problem??