I've got a 67 Dart with a 383 that has been professional built and I had an intake valve on the number one cylinder break right at the valve stem keeper. What the heck would cause something like that? Thanks Guys

Coil bind and bottoming out the spring on shims or Valve guide can do it!

were the valves replaced with one piece stainless when the motor was rebuilt? factory type valves are 3piece.

I had the valve springs pressure checked and they came in at 125 psi which should be enough for the 284/484 cam I am running. Just fyi. I got marks on a few pistons where the exhaust valves have hit them. The motor was professionally built and the piston to valve clearance was o.k. I suspected the springs are bad even though they check out o.k. I guess I should try a heavier spring. The valves are stock not stainless also.

Not talking about pressure. Talking about the travel of the spring. There should be at least .060 more travel in the spring than the full lift of the valve to the point where the spring goes solid. Preferably closer to .100 more travel. Sounds like you possibly could have floated the valves too. Actually the spring should have had enough travel in them but if you floated them that is a different story. How many RPM's were you trying to spin??? Or did you have shims installed under the springs to increase the pressure?? Shims are to be used to adjust installed height not to increase spring pressures.
BTW this was caused by coil bind.

You guys have been a huge help. Before this happened I was having problems with it, it would stop pulling at 6000 rpm sometimes in first and sometimes in second gear. It wouldn't miss fire or anything it would just totally stop pulling. I thought I had a fuel pressure or ignition problem but after braking this valve and seeing the marks on the other pistons where the valves have hit them I must have been floating the valves. The valve springs are Mopar performance springs that where matched for that cam back in the day but even though the motor only has about 1000 miles on it the springs are 20 years old. They didn't sit with rocker arm pressure for that long but with this happening and there age factored in it I will change them. Plus I don't have a lot of faith in some of the Mopar performance parts. Thanks again for all the help and if anybody has anymore wisdom to share feel free to chime in.