I have a '96 M40D that starts, idles and generally seems to run fine but when I run it at high throttle or WOT for an extended period of time it will all of a sudden bog and slow down. No alarm. Tell tale shows water flow. I have to put it into neutral for about 10-20 seconds, then I can throttle it back up to plane. But it will repeat this after 5 or 10 minutes, again only after running at higher speed. I thought this might be a fuel delivery issue but I had replaced all the lines, primer bulb, in line fuel filter, and connectors. Doesn't seem to be overheating. Is it going into a safe/slow mode? Any ideas for remedies? I've also recently changed the water pump impeller, but could it be thermostat?
