340 vibrates

My fresh rebuild has a vibration that starts at around 2k rpm and up. In neutral or park If I hold the rpm it comes and goes. Run it up to say, 2500 and hold it, its shakes pretty good, hold it for 10 or 15 seconds and it goes away, keep holding and it returns comes and goes etc. etc... It did the same thing before for rebuild but was hoping it would be gone now. It is a 72 block, forged crank that was just turned, motor balanced, new motor mounts, trans mount, converter rebuilt and new flex plate, both neutral balanced. Neutral factory balancer. Thanks in advance.