Just throwing out some ideas for you.
Like others have said,You need to read your plugs and while you have them out gap them a little tighter and see what happens.
What model number MSD are you running ? (Some earlier boxes had a problem with 6 cylinders odd fire and even fire,,,If it is a 6200 you are good)
Check the ground connection for the MSD ignition box, If the Heavy Black wire is not connected directly to the battery negative terminal it can cause a breakdown in the ignition at higher RPM.
I know you had the problem before but what if the Valve springs fix the original problem and now you have a high rpm cross fire....
Good luck and curious to see what is wrong ?
