tunnel ram tunning help 408sb

i think i would translate it into the engine breaking up at higher rpms:)

i agree with 340_8bbl on this one, what kind of ignitionsystem and have you checked all the wiring,connectors and grounds? what kind of fuelpump are you running?

third option would be valvefloat,but i realy doubt that one,since you wouldnt get it to 6500rpm if the valves start floating at 5000.