Since it is getting cooler in the late afternoon and night time I turn off the a/c and turn on the vent. I have noticed that when the a/c is off, and on vent, the car shakes about, enough to make my keys sway and the RPM meter moves up and down a little bit.... whats causing the problem.

 

could be a number of things....check your iac

 

Check your Vacum Lines

 

How far do the RPMs drop? Mine does something similar but it drops down to like 500 RPMs (and then really shakes like the car is going to die), then a second later it kicks itself up to 700-750 RPMs and its fine. I have no idea why it does this but it annoys me if anyone knows.

Matthew