Do your rocker arms have posilocks? When I first installed aftermarket rockers I didn't properly understand how the posilocks worked, and thus my rockers all started to chatter and become loose until I read about how to adjust and lock them down.
They look like this:
If memory serves, you adjust the rockers to their proper tightness, then crank down on the inner lock, and then do 1/4 or 1/2 turn on the outer nut. This locks them into the proper tightness.