I have an elx and I had the same problem 2 years ago. DO NOT let the temperature go to RED otherwise your cylinder head might need skimming and this is a costly job (when taken into account of an engine stripdown etc).
The fault was the water pump on mine as water was was leaking from the pump and it cost me quite a bit after they did the skimming etc. The pump was cheap iteself.
Also, 2 weeks after that it did it again and a pipe valve for the cooling system was letting in air and thus it was losing compression. The garage had overhauled all that and still wanted more cash from me which I said "No Way" in a polite manner.
One last thing, do not do what I did and take it to a Punto Dealer, they charge the earth, plus about 10 more planets on top.
Why it heats up when you slow down or idle is that the pressure in the cooling system drops when you idle. This results in the engine not being cooled as much as when you are travelling along.
Your fan will also sound as though it is about to take off as well.
Anyway, this might be the answer to your question.
Another Punto Owner, September 2007
