my 2004 kia sedona 2.9 deisel sometimes wont start it just clicks, ?

My 2004 kia sedona 2.9 deisel sometimes wont start it just clicks, I try to start it a few times and then it starts. I have taken the starter motor off and cleaned it, oiled it and put it back on, it was ok for a week but the problem has returned. I don't know if the problem is the starter motor or something else. Please can anybody shed some light on this. The car's done 89000 miles