Stalling on hard braking - 86 Z28

Okay, anyone have this problem? I can sit out front of my house and get the car tuned just fine, it'll idle right at ~700 rpm. Bump the throttle, revs fine, comes back to 700 rpm. However, go take a test drive and as soon as you make a quick stop or a hard turn, the car can't maintain idle speed and dies (makes for some good exercise trying not to have an accident when the power steering goes away!). The gas mileage also is pretty rotten now, only around 15 mpg (highway). I had similar behavior with my original carb too (except that one leaked around the bronze throttle bushings too), so I don't think its a carb problem.

Here's some stuff I noticed:
- I tried a fast stop with the car in reverse and it didn't seem to want to die.
- If I sit with the front of the car down a hill that the engine speed drops about 250 rpm (if that helps).
- To make the car somewhat driveable (no stalling), I've got to keep the idle set between 1250 and 1500 rpm.

Does this sound like a fuel pressure problem? Too high? Too low? Maybe as simple as replacing the fuel pump?

I've got a fully rebuilt 305 (lightly modified, bored 0.030, hotter cam, etc.), A T-5 tranny, Edelbrock tubular headers and catback system, and a new Quadrajet carb.

Thanks ahead of time for any help!