Strange one on my rb20det s14 as it drives fine at normal speed but once you get up to 65/70mph it just starts back firing whole car shakes back and fourth car just does'nt go any where.
Once you back off it goes back normal again. Does it on and off boost as can even slowly get up to that speed and it will do it.
My mods are just exhaust system, fmic, walbro.
Here a quick vid hope link works, the speedo rev counter are out due to the rb conversion.