Most likely either the chip in your key or the contacts in the ingition
switch are worn.  If the chip in the key is work the easiest fix may be
a new key.  If the ignition is worn you can either replace the switch
with it's contacts, or follow the wire down, measher the resistance of
the chip in the key and put a resister of the same value across the line
 coming from the key (cut the wire and put it across the wire coming up
from the computer) to make it always think that the proper key is in
place.  Personally I would fix the problem just to keep things as close
to original as possible.