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Speedometer bouncing with rough idle

I have a 2003 Ford Taurus V6 OHV. I noticed over the last 4 months that at intersections or when parked the speedometer bounces all over the place as well as a rough idle. Also while this is happening, if I turn on the defroster, A/C and sometimes the head lights the car shuts off immediately. I got my car tuned up around 6 months ago. Ford tells me faulty spark plug wires but can’t confirm a diagnostic test on my car because this issue doesn’t happen consistently with no “check engine light”. I want to be sure it is not just a electrical short some where.