The above three answers are baloney. One thing you can do is check whether the radiator fan or AC pump is turning on and off when the AC is on. With the help of a second person go out in the driveway and open the hood---start the car----turn the AC on and wait until the symptoms start. Watch the radiator fan and see if it is turning on and off with the symptoms you are hearing. If it is not the radiator fan watch the AC pump itself. It may be turning on and off. Either will cause the engine to slow down a bit then come back to idle when the additional power required to run the fan/AC pump comes on line. If either are coming on intermittently you will need to have a switch replaced.

Because the engine is crap and the overload of the compressor is taking out of the engine what it cannot handle

check the condencer o make sure it is properly scrolled inplace

Engine is small and now after you put some mileage on it doesn't have same compression as it had when it was new .

The shake is every 2 to 10 seconds, darn short each time, and hardly strong. (If it happens when I'm driving then it would be hard to notice).

It definitely does not happen when in park or the AC is off.

Three weeks ago I got a rebuilt transmission.