You can support the top of the beam by using 2x wood blocking that is lag bolted to the floor joists. The Journal of Light Construction (look up their website)has a nice article of this topic.
Based on the description you have provided, my opinion of the cause would be truss uplift. The metal plate connected wood truss industry has information on this subject and methods to control it. Unfortunately, after everything is constructed it is much more difficult to do. They say that truss...