Have you tried replacing the power valve?  I had a similiar problem
with mine and it lead me to this valve on several occassions.  I ended
up scrapping the Holley and going with an Edelbrock and the problems
never returned.

First, the Holley tunig manual would have a wealth of info
for you.  If this is happening as you describe just
"crwoding" the throttle, I would suspect the fuel level is a
bit low.  At 1500 cruise, you are not quite high enough to
be bringing in the secondaries just crowding the throttle.
You may want to block the secondaries temporarily to see if
this makes a difference.  If it does, the secondaries are
coming in too soon.  A power valve with too low a setting
will also cause this.  Generally, I like a power valve about
2-2.5 below cruise level vacuum.  If you are cruising at 14"
with everything running, I would try a 10.5 or 11.0  The
number is the vacuum at which the power valve begins to
open.  Most carb come out of the box with 7.5-9.0 IIRC.  You
can check the parts manual for you part # to see which it
came with.  You should also check to be sure you are not
getting too much ignition advance too soon which can appear
to have a leaning effect for the EG reader.  You need to
start with the fuel level and make sure fuel pressure is
good.  Low fuek pressure under load mau be caused by a
restricted fuel supply like a clogged filter or a defective
tank venting system.

Good luck

Lugnut