My guess is that you have either fuel or spark starvation. After driving and condition exhists is the coil to hot to touch? If so is there a ballest in the power line? Some HEI ignition systems require a ballast resistor on the 12V feed line.
As for fuel well like Joe H said, install a fuel guage and look at fuel pressure after condition exhists.
Adjusting idle mixture. Do you have a Uni-sync to sync the carbs.? You should adjust the carb.'s so that they both pull the same vacuum at the intake of the carb.'s. This device measures the suction at the carb. for each carb.
They both need to be the same before you adjust the mixture screw. Then forget the vacuum guage and screw in (one at a time) the idle mixture screw untill a noticable drop in engine rpm is noted, turn it out to its best idle and add 1/2 turn. Do this on both carb.'s one carb at a time.
