Take a guess where the bottle neck is!

Valve springs use shims to correct the installed height of the spring.

When the valve is in the closed position,the valve spring is still holding a certain amount of pressure on the valve to keep it shut.
The amount of pressure the spring is giving to the valve retainer in this state of being closed, is determined by it's installed height.\\\\ This is the amount the spring is being crushed when the valve is closed.

If you have a valve job done on a set of heads,the valves will be sink in the head a small amount.
This makes the installed height of the spring increase and become taller.
This in turn gives you less spring pressure when the valve is closed.

If you do not make sure that the installed height,after a valve job, is not correct,and you do not have the minimum crush that the springs where designed to have,you will not have the correct holding power for the spring to work through the entire rpm range.
The result is VALVE FLOAT. OR A BETTER WORD IS>> VALVE BOUNCE.


I am not suggesting that this is your problem,but it could be.
Ask the person who did the heads if he checked the installed height when he did the heads.
I can't see a valve job being done without shims, unless bigger valves where installed and the minimum crush in foot pounds was good.

PS>>>>>IF IT IS THE VALVE SPRINGS,IT WOULD HAVE ALWAYS DONE THIS FROM THE DAY THE HEADS WHERE INSTALLED ON THE ENGINE WHEN GOING PAST 5000 RPM'S.
You said you have 20 thousand miles on the heads.
Valve springs last almost for ever in mild performance engines.
Think of it this way.
If it's a NEW PROBLEM, then it's most likely NOT A STEEL PART.
It's in the IGNITION OR CARB.
GOOD LUCK.........:thumbup: