NA engine vibration problem

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Guys, I have a 93 MK 2 with a NA engine. The car has 125,000 miles on it. I only drive about 2000 miles per year. A few years ago the engine started to develop a vibration that increases with RPM. I had a major tune-up, cleaned the fuel system several times with in-the-tank cleaners, gone to several auto repair dealers, and no one has a solution. The problem seems worst when it rains, but all the spark plug wires and dist. cap are new so what