How much experience do you have with this sled, and has it ever done this before, even if only once or twice?
You might have air in the system, could try to bleed, but I would start to suspect a water pump issue. Others have more experience with overheating situations, but on the water pump subject, you may have an impeller issue although if the splines are stripped I wouldn't think it would have the power to push warm/hot coolant into the exchanger.
A more likely water pump scenario is that some of these engines experience the brass drive gear on the crankshaft slipping when the engine warms up. The gear is fine with the engine cold and early in the warm up, then with the increased heat to full warmup, the gear expands just enough to spin on the crank, and the coolant pumping stops (as does the oil pump since it is driven by the same gear). You have two things happening that point to the gear, 15 min into the run the engine is warmed up, and you have heat in one exchanger so it does, or did, pump. I bet if you checked your exchangers at 5min and 10min, or even closer intervals, you would find both exchangers warming nicely, and then one starts to cool...
Not what you want to hear of course, because it involves either 1) replacing the crank or 2) sending yours out to be fixed.
Just food for thought, others will chime in hopefully with better and less expensive ideas. You can also search these forums for more on the overheating subject, you'll likely find enough reading to keep you busy for awhile, and give you plenty to investigate.
