Tractor Repair: Ford 3600 diesel 3cyl 8up/2bk-losing power, blown head gasket, tiny air bubbles

Question
I have a Ford 3600, diesel, 3cyl(?) with 8 forward gears and 2 reverse(hi/low). I hope that is enough info. The problem I am having is when I am running the bush hog. The tractor starts with no problem. I can sometimes cut for 3-4hrs with no problems and sometimes only get about 45 mins before it suddenly loses power and starts smoking(white smoke) like crazy. The engine still runs but it's like it only runs on one cylinder or something. (sometimes, not always, right before it happens it will sputter so I know something is about to occur.) I usually drop the rpms, shut off pto, and drop to a lower gear while attempting to crawl to a shady spot. It will crank right back up, but I like to let it sit for about 10 minutes. Once started up again I can only resume cutting for about 15mins before it happens again. The longer it sits, the longer I can cut. I checked all the fluids and they are all full. The only one I was a little concerned with was the hydraulic fluid right infront of the h/l shifter. It is white(ish# and has what I think would be tiny air bubbles in it. ??? Could that be an issue? I don't have any idea what could cause such a problem. I really know nothing about tractors except how to work them. Can you please give me some idea of what issues I should check into? Do these tractors have a sensor to shut down if they get hot? (gage never reads hot).  Thank you very much for your time.

Answer
  This one is a little weird.  If it was starving for fuel, it would lose power after working it for awhile, but would not make white smoke.  If the air filter or intake was plugged, it would lose power, but would smoke black.  If it had a blown head gasket, it would smoke white all the time, not just at certain times.  It sounds like a possible timing advance problem inside the injection pump.   Check to make sure there is a good flow of fuel from the tank anyway.  If the fuel filters have not been replaced in a long time, it would be a good idea to change them also.   If the oil is white, there is moisture in it and it should be changed.  There is no automatic shut-down feature on this model to protect the engine if it gets too hot.