RPM's cut our holding speed

[Erik1371]

I'm new to this so bear with me, I have a 72 k10 with the original 350, edlebrock 1406, the original distributor with an ignitor kit and coil and freshly rewired truck. The problem is when I hold speed above 45 it will stutter for half a second, rpms drop about 1000 and come right back up, I can rap it out to 4000 rpms with no problems it only does it while maintaining speed. Sometimes it does it 2or3 times in about 10 seconds, sometimes once every few minutes and sometimes not at all. Any Ideas?