MY 1992 Subaru Liberty has a habit of getting the shakes. It doesn't happen often, but when it does it feels as if it is running on only two cylinders. I have had the timing belt and water pump replaced, and while accessing this area of the engine all other parts that might fail were replaced, including plug leads and coil.
IF YOU'VE checked the electrical system I would concentrate on the fuel system. Check the fuel pressure and fuel-pressure regulator, and make sure the injectors aren't clogged.
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