engine stalls under load at certain speeds

I have a rachero with a 289, and a Carter Carb, Hei and a few other hi po parts. However, lately it seems to bog down and cut out..very similar to running out of gas. and then will go back to normal if i slow down or speed up. but this only happens when I accererate at around 35-40 MPH (medium pressure of course). If I accelerate slowly, it is o.k., If I accelerate hard, it is ok. This is only when the temperature is not quite up to normal. after it is has been running for a while it goes away (I live in a small town so leaving it on isn't practical). I had the timing and carb adjusted, cuz that seamed like a reasonable answer, but of course it was warm and the mechanic didn't seem to understand it only happens when it is cold. ( I left it parked in front of his shop the next morning and he still didn't get it).

Any Ideas?

Sounds like the choke is coming off just a tad early. What choke set up do you have on the carb???