Engine Performance problem
1999 Pontiac Grand Prix 6 cyl Front Wheel Drive Automatic 88888 miles
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When driving the GP it intermittantly chugs, IE going 35-45 mph the tac will jump approx 1/4 inch up and down and the speedometer will jump around also. Going 55-65 mph it does the same thing. I have replaced the throttle sensor as advised by dealer. I have flushed and changed filter of transmission and added 1/2 can of sea foam transtoon solution, none of the above has helped. please advise, thanks