If you experience high load misfires (but revving fine to the limiter under no load conditions) be sure to check the injector duty has not gone over 95% as the ECU auto protects and turns off injector outputs should this happen. If you need 95% duty to get the right AFR, then it is time to install bigger injectors! Of course also check you are not getting spark break down due to failed plugs or failing ignitors!