Maybe you should try and pull the codes when that happens? http://www.allpar.com/fix/computer-codes.html
EDIT: and just to cover the obvious, your shifter is in Park or Neutral, correct?
EDIT2: another idea is to switch the ASD relay with a similar non-critical relay to make sure it's not that.
EDIT3: did some searching on web and sounds like a symptom of either a bad transmission range sensor (TRS = modern equivalent of park/neutral safety switch), a gear selector in need of adjustment (try rowing through the gears a few times to see if it helps) or other electrical/wiring issue related to the TRS.
EDIT: and just to cover the obvious, your shifter is in Park or Neutral, correct?
EDIT2: another idea is to switch the ASD relay with a similar non-critical relay to make sure it's not that.
EDIT3: did some searching on web and sounds like a symptom of either a bad transmission range sensor (TRS = modern equivalent of park/neutral safety switch), a gear selector in need of adjustment (try rowing through the gears a few times to see if it helps) or other electrical/wiring issue related to the TRS.