Q: What is causing my van to miss out at 40mph and above? on 1999 Dodge Ram Van 1500

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a diagnostics test at shop said misfire cyl 3. I replaced that spark plug, the second one back on driver side if looking from front of van, but it made no difference. The plug seemed just fine. My question is 1. Would changing all plus helped? 2. What should be my next course of action in my quest to diagnose this issue? 3. I only ask cuz while in reverse I hear a funny noise like its struggling. Could the "missing out" I'm feeling at 40mph and sometimes on accelerating from stop be a symptom of transmission issues? thanks in advance for any help.
(2) Answers
Get it scanned again and post results so we can help. Give code(s) like po303 ect. Oh, that was the # 3 plug that you changed.
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