JUST ONE TIME I WOULD LIKE TO COME ON HERE WITHOUT A PROBLEM! HERES WHATS WRONG,IM HOPING SOMEONE CAN HELP.FINISHED BUILDING A 383 STROKER.THERE IS PROBABLY 2 HOURS ON THE ENGINE,GIVE OR TAKE 5 MINUTES EITHER WAY.JUST STARTED TO SHAKE PRETTY VIOLENTLY BETWEEN 3 AND 4 THOUSAND.I DISCONECTED ALL THE BELT DRIVEN PARTS,STEERING PUMP,ALTERNATOR,AND WATER PUMP,TO SEE IF THIS WAS WERE THE PROBLEM WAS.NO. IT DONT MATTER IF ITS IN GEAR OR OUT,NOR DOES IT MATTER IF ITS UNDER HEAVY OR LIGHT THROTTLE. THERE IS NOTHING VISIBLE THAT WOULD LOOK LIKE THE PROBLEM.ALSO,THE ENGINE IS NOT MAKING ANY UNUSUAL SOUNDS OR NOISES. GUYS,IM STUMPED!!! COULD IT BE THE HARMONIC BALANCER,I PUT A NEW ONE ON AT REBUILD,IT IS A STOCK 400 8 INCH REPLACEMENT.I ALSO PUT IN AN SFI RATED FLEXPLATE[FLYWHEEL].???????HELP!!! THANKS IN ADVANCE!!!