Though I have never had that problem with mine (yet) I know I have seen several threads dealing with it. If you haven't already done so, you may want to try searching for tach and tachometer. Also, the FIY and Skinnerbox threads may already have an excellent article covering the topic. Best of luck.
Steve

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Steve
'87 300TD - 132K - Soon 4-Sale
'84 300D Turbo - 122K - Driving
'77 VW Type II - 77K - Restored
'08 250EX Ninja
English Bulldog (Brier) - My best friend. Passed away 12/02/04 while in my arms.