A drop of 200 to 300 rpm is pretty normal. Anything past that is not. An rpm drop with shaking can usually be traced to either a fuel mixture (too much fuel, or lean mixture from a vacuum leak) or, an ignition misfire (ignition is actually suppressed when the engine is under load) Therefore weak spark (bad coil), fouled plugs, a bad cap, rotor or ignition wires, will not permit good spark inside the combustion chamber, resulting in a mis-fire and shaking..
There are other larger issues that can also cause similar problems such as a burnt valve, stretched timing chain, low engine compression or a mal-functioning transmission torque converter. Be aware of those things but don't worry about them. 99% of problems such as yours are isolated to what I've written.
in any computer controlled system, guessing is not an optional means of diagnosis. Other than obvious problems that you can see (like a vacuum hose that is cracked or has fallen off or ignition wires that you can see sparks jumping from in the dark), you need to use a decent manual like haynes or chilton in order to do proper testing and find the problem without spending your entire bank account replacing parts.
If you have a question about anything you find you can ask here as well!
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