Sputtering at 3000 rpm's

[tino0217]

I have a 71 C10 350 sb Edelbrock 600 series carburetor. I was having starting problems just outta the blue, I did a tune up changed spark plugs, distributor cap, modulator, coil, mechanical fuel pump. I timed it to where it sounded fine no backfiring but when I get it up to 3000 rpm's in 2nd thru 4th (granny low) it sputters and bogs until I shift to the next gear and runs fine accelerating until it gets up to 3000 rpm's. I went back to the way it was when I bought and disconnected the vacuum advance and disconnected the auto choke but still the same. I've never had this problem since I bought it 5 years ago. Anyone run into this problem. Any suggestions?