Not sure how to describe this problem..

I have a chunk of code in a webpage that works like this, to rotate some ads.. : if ($row = mysql_fetch_array($getads)) { do { $POS = $row[LASTPOS]; $ID = $row[ID]; $POS++; if ($POS > $numads) { $POS = 1; } $update = mysql_query("UPDATE adtracker set LASTPOS = '$POS' where ID = '$ID'"); echo $row[LINKLOC]; } while ($row = mysql_fetch_array($getads)); } so it grabs the last position that ad was in from the DB, increments it by 1, unless incrementing it would push it past the max # of ads, so it resets the position to 1. The problem is, the site gets a lot of traffic, and instead of the ads retaining positions like: 1, 2, 3 or 2, 3, 1 or 3, 1, 2 as they should, I've seen it somehow go to: 1, 1, 3, or 2, 2, 1 etc. So I guess what is happening is that when 2 people hit the page at the exact same time, it's getting updated values out of the DB for some of the ads before they are all updated.. how can I go about controlling this? Thanks, Chad