Dropping Valve Seats?

[Cobra75]

Last fall, CSX4100 was running on 7 cylinders...it turned out the intake valve seat on #1 cylinder had dropped and the intake valve was stuck open.

Well after last month's SAAC event in Nashville, a similar "missing" started to occur again. It got worse on the return from Cobra Revolution, and two miles from home, a pronounced clatter began. I immediately shut the engine down and trailered the heap home. This time the noise appeared to be coming from the other side. Upon removal of the driver's side valve cover, I discovered #5 intake valve in the open position. This time the valve spring had broken, and the keeper & retainer of course were loose on the top of the head.

The similarities are striking:
Both times it was the front cylinder.
Both times it was the intake valve.

What could be causing this? These are Edelbrock Performer RPM aluminum heads, complete with Edelbrock valves, springs, dampers, retainers etc. They have been installed since 1998. No porting work was done to the heads. The engine never runs hot, I have a rev limiter set at about 6000, and typically short shift at track events. So I don't think I abuse the engine.

I have never heard of a valve seat dropping out, much less two of them. Pulling the heads requires the removal of the engine due to clearance issues. The last repair cost a fortune due to the labor-the machine work and parts were cheap.

This time, I am thinking of pulling the engine myself and taking it to a shop and having it torn down & checked from top to bottom and dyno tuned.

Any ideas and/or advice?