2005 Impala....Car started to vary in engine performance

The car started varying in engine performance as in running normal for a bit then act like its running at 80% not really dieling all together but noticable. Also the speedo will start bouncing around when this happens. Any
ideas I did run some injector cleaner (no changes)and I will change the fuel filter next.
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Mike Sanford wrote:

Make sure the engine and ecu are grounded very well. Check the alternator for a bad diode.
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