An engine is designed to balance itself when all cylinders are running. a 4 cylinder powers at either 90 degrees or 180 degrees depending on its design. 6, 8 10, 12 cylinders are designed the same way, but the degree which they fire is different, but they still do the same thing. without looking at your engine, it sounds like 1 or more cylinders is not powereing properly. this could be caused by a fouled spark plug, a bad distributer wire, or a bad fuel injector. this is only a few of the things that could cause this. my advise is to take to a mechanic that you trust, (ask your friends for references), & ask him to do a diagnosis for you. he will probably charge you for this. with newer cars, this is perfectly acceptable. however, if he comes back at you with"it just needs a tune-up", ask him how he determined it, & what does he include in a tune-up. a proper diagnostic should cost about 100 dollars, but it should tell you just about everything you need to know about your engine, bad timing belt, clogged injectors, etc. some shops even absorb the cost of the diagnosis if you let them do the repairs. hint: if it only shakes on a wet day, or smoothes out after it warms up, its probably plug wires or distributer cap. a good way to check is wait for a hot dry day when the engine is running smooth, take a windex type spray bottle with water, & squirt the wires & cap while the engine is running. if this is the problem, the engine will start shaking. end result, try to find a mechanic that you can trust, & then trust your mechanic.