!. Check the voltage of your starter battery (the big on under the hood). A weak battery can cause all kinds of wierd electrical gremlins.
2. Check and see if your liftgate closes properly on the latch. If you go over a bump slowly, such as a speed bump, and hear it knocking around, you hatch is loose and is causing the alarm to sound whenever. Mine did this and I fixed it by wrapping a few layers of tape around the latch post. No problem since.