I am experiancing some troubles with my 23 year old Yanmar 2 cyl. diesel. During operation, the rpm''s occasionally fluctuate and drop then it resumes its correct rpm again. It does this for about 15 stressful minutes before again returning to its proper rpm. I think that maybe the fuel pickup is clogged or something. It seems to be definatly fuel related. I have clean filters (racor) on the fuel system. Is there a way to clean the tank and fuel?