I have a Trek "Mountain Trek" bike. Recently, the seat post keeps loosening and then foom! The seat post shoots down the post tube.

I check the seat post collar lever-clamp and it is still tight in position. That is, it didn't somehow pop open or come loose.

To remedy, I loosen the lever-clamp give it a couple of clockwise spins, and push it back down in to tight position. A couple of blocks later, the seat starts rocking side-ways and I know what is coming next; FOOM! Again.

Any ideas what the heck is going on?

Mike