Get's up on plane no problem but loses RPM's and sputters as it bleeds back down to idle speed. When cold, takes 3-5 minutes of running for this to occur. Once it loses RPM's, it will occasionally come back on plane momentarily, before losing power again. However, it will run at idle speed perfectly. I have replaced the high pressure fuel pump and drained the fuel filter when doing so. Any ideas would be appreciated? Pat