Obviously something wrong with the clamping mechanism or adjustments and the bike should not be ridden until repaired.. if this were a common occurrence, there would be lots of disgruntled owners and certainly a recall of the mechanism by Tern .. as neither of these have yet to occur, I would contact Tern and have them recommend a course of action (probably to take it to a Tern dealer or authorized repair facility for warranty work) .. don't ride the bike until it is fixed.. Tern has a forum also ..
http://www.ternbicycles.com/us/forum