Phantom fluttering

[custom10nut]

I took the truck out for a ride to my favorite vintage car lot today. While driving at cruising speed, I'm getting a "fluttering" feel. It's not really a hesitation, as the tach is staying at a steady rpm. When Iaccelerate the flutter goes away until a few seconds after I level off the speed.
These are what I think are the possible causes:
Ignition: I am using a Mallory electronic conversion in place of points.
Carburetor: I'm running a new Edelbrock 1406 that has been tuned
Transmission: about 5000 miles ago, I had it "rebuilt", but the guy only put a new clutch pack in, without replcing the front pump, he told me the pump and torque converter were "fine". Now I'm beginning to wonder
This phantom is really starting to frustrate me.
Anyone out there have any suggestions?