My truck has had a misfire ever since I got it tuned. Basic 4.8 with longtube headers, and a street tune. It has replacement stock plugs and msd wires. It had a constant misfire under 2k rpm, and only under a moderate to heavy load. I put the stock wires back, and it got bad enough to throw a random misfire code. Could my plugs be bad? There's no vacuum leaks, and it runs fine on the top end. The misfire while starting to move and at cruising is driving me crazy. Any thoughts what could be wrong?

 

Plugs gaped correctly could be a a bad plug I've had new ones go bad before