I have had the same problem and after tracing the cell I found that one mobile requires 10sec for a location update and it keep asking every minute, normally it requires 2-3sec ask every 1.30h. I thought it was a strange mobile so I put him on trace but the next night it was ok. than another mobile on another cell was having the same problem. This problem does not happen very often on our network so its not a problem, but Im curious to know why?