Okay heres the deal, I have a fairly new SBC 350 slightly modded with an Edelbrock performer intake, headers and a rebuilt quadrajet. The deal is that I'lll be cruising down the freeway around 65MPH and I'll open up the secondaries and the engine will start making a popping noise (backfiring/too rich?) at the same time the engine shutters slightly, It only seems to do this when im around 65-70MPH. On the other hand I can be cruising around 45MPH with it whining at 5000 RPM with the secondaries open and it performs top notch. So I don't think its related to engine speed.

EDIT:

It won't shutter/pop at anything slower than 60MPH

And if it matters Im running a 700R4 tranny.