mine did the same thing and its most likely a problem with the torsion bars in the trunk. if you look where teh bars attach, there are a few slots that they can use to put more tension on the hinges, thus keeping the decklid up. it is possible to adjust the bars with a pair of Vice Grips, but its kind of hard to describe in words. maybe when i go and buy new batteries for my camera i can try to show how. i know there is another thread about this somewhere... thats where i learned
The trunk lid has 2 rods that act as springs. On each side you'll see the bracket the rods connect to. They have 3 slots, one is where the rod is now, move one rod one space, by using a good stick, like a broom handleand prying. Careful, this can be dangerous! Maybe someone has a better way of doing this, as I did mine 7 years ago when I mounted a luggage rack on the decklid., so my memory is a bit faded.
Notch toward front of car (back of trunk?) is lightest load.
You can set the bars unequal by one notch - but you should not set one in the front notch and leave the other in the back notch. The best thing is to equalize them if possible. Remember, the more tension you put on them the faster they will "wear out" or weaken again.
I use a pair of vice-grips to do mine.
Be VERY careful! These things have quite a bit of energy stored in them. If you are going to get in the trunk (some folks find it easier to do it from inside) make sure that you disable the lock so you don't get stuck in there by accident!