My Tarantulas does that. He could be there for days or weeks and suddenly BANG! the missile fires and scares the crap out of me.
I have had to replace the spring of several of my G1 figures and added springs to figures which springs were removed for safety reasons. The best springs you can use are those from those multi color pens. I buy them from the reject shop, remove the springs and cut them to the appropriate size if necessary.