Start by checking that your plugs wires are fully seated on the top of the plug. To test this, you can pull the wire completely off the plug and put it back on (which should reward you with a satisfying "click" or other sign that it's all the way on). Also be sure they are on in the correct order, especially if they've been replaced recently. Next, make sure moisture hasn't made its way into the spark plug "wells". If it has rained hard recently, or you've gone through a car wash, poor-fitting or worn plug wires can allow water where in, and moisture will kill a spark quick. If all that does indeed check out, try replacing the spark plugs and spark wires and hold off for now on the distributor cap. New plugs and wires may be all that's necessary to cure the misfiring. But, if this doesn't correct the problem, then go ahead and replace the distributor cap. If you don't want to do the work yourself, you'll need what most garages call a "maintenance tune-up". This should cover the items mentioned above, depending on your engine and general factory recommendations. If the vehicle still shakes even after replacing the distributor cap, you could have a burned valve. Take this vehicle to a garage to confirm this and have any necessary work done