Timing Retarding @ high RPM

Hey guys Tring to track down a timing problem 4000 rpm and up.
408 with Holley dual sync, hyperspark coil and CDI box. Distributor is installed and phased correctly. Timing at idle matches perfect.
Now I've been trying to figure out a misfire/stumble that happens above 5800 rpm. I Thought would double check my timing at higher RPMs. I perform a static timing check @ 34degrees ( For you non EFI guys this is like locking out your timing) and it stays pretty steady with minor fluctuations but once I get to 4-4500rpm in drops to 30 degrees. No matter how much I adjust and add to my inductive delay I still get that drop and timing retards. If I add too much inductive delay , the RPM below 4000 it will advance the timing as it should but once it gets above it retards back to 30 and fluctuates.
Any ideas?