I have a 2008 Suzuki 60 hp 4-stroke. I run it around 5200 RPM, not wide open but not far from it. I've had a problem for about 6 months where after about 5 mins running, it will bog down, it was dropping 500 RPM but it would pick back up but I couldn't get anything more out of it. After it has run when I idle to lets say check for a spot to fish it idles rough and usually cuts off and hard to start. After it has been cut off to fish, it idles fine. Plus it has used some oil during this and there is a fuel/oil type stain at the motor while I'm on the water. If I run around 4800 RPM it won't do this. I have done the following:

changed low and high pressure fuel filters
cleaned VST needle valve, screen on high pressure fuel pump
changed fuel lines
cleaned fuel injectors

I really don't think it is anything internal like valves but is it possible the low pressure fuel pump is the problem, either the diaphram is going bad or it's just breaking down?

Thanks!!