Om617 overheating issues??
 

So this is a swap we did in my 95 land rover disco. Anyway around town it only heats up to around 180f. But the other day well sitting in traffic and then hitting the HWY and going up grade it started to get hot. Hit the top of the hill going 50mph and started down and it got to around 240f and then the motor just cut out and dropped all power wouldn't even idle.
Coasted to the side and let it cool down,after a few minutes it would would turn over but wouldn't idle or stay running. Finally after some time it started to idle and I was able to then drive it home with no issues .
So I'm scratching my head on this one,why would it heat up a point that then motor just dies?
Thanks for your help.

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Oh and it's an auto trans,using the land rover v8 rad which is in good shape
With a Manuel clutch fan,and the water pump was replaced less then a year ago.