I had this problem with mine when i bought it.
First thing i would suggest doing is pulling off the engine cover, not hard to do. Then check that either the engine cover has all the mounting points all intact and have all 4 rubber grommets that sit in the engine cover and then push onto the previously mentioned mounting points. Making sure these are all in place will help certify that your engine movement isn't causing your engine cover to work its way off and then causing the air intake cap that you mentioned to work its way from the mounting position.
To go even deeper into this you could reduce engine movement by looking at the engine mounts such as the doggone mount which i believe is the one that helps most with engine movement? Correct me if I'm wrong..
Hope that helps. I had the same problem with mine but my engine cover was also cracked in a few places and had 2 out of 4 engine mounting holes cracked which then obviously worked its way free end creating the problem with the air intake.
