When I brought my recently purchased escort cosworth out a few weeks ago, it functioned properly. When I turned on the boost this week, it spluttered as I drove it home. I went to Ford and placed an order for 071 plugs with a 0.6mm gap. After that, when I took it out, everything ran smoothly, but after 20 minutes, it began to splutter once more. Any recommendations? The car is standard save for an exhaust and chip.