Hi Brian
You have 2 things I think you can check===Are the 2 electric fans running all the time keeping the radiator too cool.????
They are controlled by a thermostatic switch.??? That would perhaps explain it getting cold after a short time.???
The other thing I think you should check is the pressure-type filler cap in the expansion tank --If this is faulty the
water in the engine will not circulate correctly.???
Just a though I hope it helps.
Frank