There is a thread about it somewhere on here....but to give a quick answer... I bought a Cherokee diesal in the UK , intending to drive it back to Poland , and found to my dismay i had the same problem as you....
In desperation i chucked in two bottles of steel seal and headed for Poland... the tempreture stayed at normal all the way ( 650 miles ) and since then i have done thousands of miles with no problems including several long trips to Ukraine... so i would definatly reccomend the stuff...
In a few months i am heading off the a new home in Moscow in my old oil burner.. fingers crossed..