My AC works intermittently. Sometimes it works normally, other time regardless of which speed I turn the knob to, nothing happens, and other times when the knob is on 3 or 4 the air is very weak and when on 1 or 2 there is no air blowing.

Any warranty left on the vehicle? Should be 5 year, 60,000 mile base warranty I believe? If under 60,000 miles, consult your local dealership for in-warranty diagnosis and repairs. If over 60,000 miles, and without a good relationship with your local dealership, have a local shop near you (or your dealer) perform testing on the ventilation system controls and blower motor. Testing will yield an answer as to what is going on. www.repairpal.com/repair-shops