Slant won't drive over 25mph without cutting out

I tried Pulstar plugs in my GTO. Noticeably smoother idle. But then one failed......
Pulstar is on I think Gen 4 of their plugs. I got hooked about 14 years ago. They went to Platinum and Iridium, which totally sucked! They recently (within the past 5 years) did another revamp. I have the current generation in our Jeep GC and like them. Standard plugs kept fouling out cyl #1. These keep firing.