then check wheel balance
It could be your alignment. If it's off it shakes.
If the vibration is from about 45 mph and faster the tires are out of balance or bad.
If its vibrating at a slower speed you have a bad suspension part and if it vibrates at idle you need a diagnostic and possibly a tune-up.
First check the balance of your tires. This is the cheapest scenario. Then the suspension and allignment.
More advice:
you could also have a separated or broken belt in the tire
Yes, either can cause a shake while driving.
Loose motor mounts.
Nope.
Most likely it has unbalanced or bad tire(s). Bent wheel rims also can be the cause.
Sounds like wheel imbalance or possibly a bad wheel bearing(s).
If it only happens when you are braking I would suspect warped brake rotors. Other reasons the front end could shake would include unbalanced tires or the vehicle needing realignment. Whatever the case may be; driving a car in this condition is unsafe and you should repair it or have it repaired before you drive it again.
Out of balance or damaged tire(s). Damaged wheel(s). Engine misfire. These are the three most common culprits.
spark plug gap Can also be a vacuum leak.
i would really like to know myself
It is important to maintain speed while driving, unless coming to a stop. A car would decelerate if there was not enough fuel or electricity to keep the car running.
Normally this is caused by warped brake rotors.
When driving the IAC does not do anything because the car is not at an idle