If it worked fine at one point, I'd say either the tach is broken or that there is some sort of signal problem to the tach. Look at the factory tach and tell me what it is doing when you have the problems.
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I X L - Doss | 95 Cobra hardtop convertible with a few mods
For sale: 4 or 6 pt unwelded cage - pick up preferred | '99 YZF-R6 | '02 Audi A4 1.8t quattro sport | 2005-07 Saleen parts