First my sympathy, I've been there....three times! That might seem a bit stubborn/stupid but the problem was rare and intermittent so each time I fixed one thing and thought the problem was solved. When the bike is cold the oil was thick and pressure great, the problem would occur suddenly at speed. It seemed to be a loss of oil pressure to the head, resulting in burnt/worn rockers.
The repeated disasters tried my patience but I ride in Tanzania and its hard to find another bike.
The problem is not uncommon. I know 3 other XR600 riders here who had the same problem, all for slightly different reasons. It seems a weak point of this model. My problem was caused by a combination of things, that's why diagnosis was hard.
In the end I rode carrying a 12mm spanner to loosen the head oil supply line and check pressure throughout rides, and I fitted a temp gauge to watch the oil temperature.
Here are the things I checked/repaired, in the order I did them. I assume you changed the oil and oil filter in time :-)
Oil pump - open it and look at the faces of the two gears, if there is bad scoring replace the pump. They are tough but one friend managed to burn his rockers and seize his engine with a worn pump! When you put the pump back be very careful to put the two small alu bushes and their sealing o-rings back in and the correct way around. Also prime the pump by filling with oil, it does not prime itself. Always after reassembly open the head oil supply line and kick until oil comes, to make sure its working!
Oil Strainers - Clean them both. There is one in the sump, accessed by removing the clutch side engine cover. The other is easily missed (and just cost another friend his rockers) its inside the frame at the bottom of the front frame member. Disconnect the pipe that supplies the pump with oil from the frame, and screw the strainer out of the frame.
Oil supply line - this is the one which I think was the primary cause for me. The oil supply pipe that takes oil from the bottom of the frame to the oil pump has a flexible rubber section. I had checked mine several times and failed to see the problem, blew it and it seemed fine. Finally in desperation I sawed through it! The outer layer of the rubber had separated from the inner layer, and under extreme heat and a vacuum it collapsed. Invisible form the outside but cutting off the oil supply almost entirely. After fixing this all was well, and then I stumbled upon a service warning issued by Summers racing about just this problem. They said the heat, and vibration and the twisting force applied when tightening the retaining nuts caused the invisible failure.
Well I hope my long and painful diagnostic process helps. Mine now runs great, a little too well in fact as I started riding like a loony and a couple of weeks ago broke my leg.... but that's another story, and the reason I have time to waste on the forums.
Hope this helps, Good luck,