sounds like a tuning issue, the car in drive and going over 45 or so is the most load at the least rpm. so either your timing is not advanced enough or like they said the carb is leaning out. stock timing is 10-12 base timing, on up. mine is about 18-20, for a pretty heavily modded sbc
good tuning procedure for holley;
Look at your primary jets. Whatever size they are, buy some that are 2 sizes, 4 sizes, 5 sizes, and 6 sizes smaller; and 2 sizes larger. You'll be needing them.
First, set the fuel level. If it has sight plugs, set the primary one to where gas dribbles out while the engine is idling, when you just bump the fender; and set the sec one to where you have to jiggle the car pretty good to get it to come out.
Lean the primary jets (using the "step size" description above) until the car just barely starts to surge while cruising (55-60 mph) at a steady speed on a level highway and it starts going up a hill. Ignore all other misbehavior of any kind for the time being. Once you find that point, go back up 2 jet sizes.
Once you get it to do that, look at the primary side power valve. If it'* stock, it'* probably a 65. Stupid. Raise the value until the flat spot goes away when you're in high gear and driving along at about 35 mph (basically, as slow as you can possibly go in high gear), and you give it gas. It'll take a MUCH higher number power valve.... probably a 105 or a 125. If it starts giving you the burn-your-eyes idle, go back down one step.
When you get that right, set the idle speed right (800 RPM out of gear, say); then get the idle mixture right by turning each pimary idle screw in until the RPM drops, then back it back out 1/8 turn. Shut the motor off, and average the 2 screws (like, if one is 5/8 turn out, and one is 7/8 turn out, set them both to 3/4 turn out), and restart the motor, and check them again. Repeat until they're equal, or until you have determined that they absolutely CANNOT be made equal. In no case should they be more than 1/8 turn different.
Take the carb off, flip it over, and look at the transition slot. You want less than .050" of it exposed below the pri throtle blades. If more of it is exposed than that (it will be), open the sec throttles with their idle screw, 1/8 turn; put the carb back on; re-set the idle speed and mixture; then pop the carb back off and look at it. Repeat until you have between .030" and .050" of the transition slot showing.
At this point, the thing should run like a bat outta hell, on the primaries. The secondaries could be just about anywhere. Put a 65 power valve in them, if they don't already have that; and select jets for max ¼ mile mph.
At that point, you'll have your Holley set up better than about 95% of all other Holley owners. You'll be getting nearly the same gas mileage as you would with a Q-jet, and you'll be making 20% more power than most cars with the same cam & heads, and FI.
1998 Bonneville SSE
2007 Monte Carlo LT-KYB, Clarion, Polk audio, Kicker
1999 Yukon SLE-KYB, Air lift, Reese dual-cam, Tekonsha
1987 Firebird-355/th400-nothing stock