I had the same problem with the bolts. I simply pulled the milled flywheel off, and balanced it true on my high point prop balancer... all the probs went away. (drill holes in the back to remove weight.)...

Sounds like you are setting up a standing wave along the axis of the crank.... I would check the balance... if this fails; try simply changing props t see if you can move the node of the wave behind or in front of the mount. If the bolts are sheering... you can bet a killer wave is being set up with your exact config (you just got lucky:-)

OR

Your steel bolts are gouging the aluminum????... put one steel and one nylon washer on each... you could be simply wearing down the aluminum during the first few mins of running which causes them to come lose , in tern, cause the vibration and a this wave.


OR

Is that a plastic four stroke prop (Dynatrust???) This prop could be flexing at higher RPMs (around 5K) and givening you this problem as well...

food for thought...
Tim