Priobe,
This sounds very similar to a problem I had a while back. I was breaking push rods and rockers at low rpm driving in traffic. Each time I took it back to the builder he blamed me for abusing the engine. What it turned out to be is the builder did not have enough clearence where the push rod goes thru the manifold. I had many pushrods that were touching the manifold. This was causing sideways pressure on the pushrods which ended up breaking many pushrods, rockers and it also broke 2 lifters.
George Anderson wrote a good article on this problem
http://www.gessford.com/projects/feintakes.htm
Dont know if this is your problem but it is worth taking a look at.
Good luck
John