Palpitations provoked by light exercise

I often get continuous palpitations (consecutive hard beats) on even light exercise, like walking.  They start when my pulse hits around 65, subside when I rest.  Been diagnosed with rate-dependent left bundle branch block (LBBB).  My doc says its very rare to feel LBBB.  I'm taking a beta blocker - it helps some, but not enough to enable me to really exercise.  Sometimes it's very uncomfortable to even walk.

Has anyone here experienced palpitations with light exercise?  What treatments have you found helpful?

Thanks.