Need some advice here on my RB V12. My engine seem to not last a 20 min main, actually it would only run consistently for 5 to 6 mins and it would just flame out. Temp wise it's only running at 115C. Tried replacing the plug and it look like it solve the problem but problem comes back again. All the help will be much appreciated

My goal is to at least finish a main
Thanks!
Roy