Replace the car's battery.

If that does not do it, we can go from there. I know this sounds like a waste of money, but fact is, most of the time, a funky spoiler is a sign of a weak battery. You can spend time and money on a lot of ideas, but the smartest one is the battery first, then go from there.

(This assumes your battery cables/terminals are not all corroded and filthy, if that is the case, clean things up first.)