i had this problem with my doblo, got it checked a the fiat dealer and they said the relay was at fault.
I think the initial fault is with the relay and this in turn wears the plug out.
My plug take about 15 seconds to warm up in the cold, im going to replace them both.[/QUOTE
my peugeot 806 i had before my doblo had this problem, i swapped the glow plugs and they only lasted 4 months, on further investigation i found even after the glow plug light had gone out on the dashboard, there was 12v still present at the plugs, so i think the relay was staying on to long reducing the life span , that and they were dead cheap, they are easy on the 806, but look mutch more difficult on the doblo which is why i have left mine dispite the warning light flashing for the last 21 months with no starting problems, even in this extreme cold weather. regards, geoff.