HELP! Miss on #6

[ndopita]

Over the past few weeks, my 1969 C10 has developed a strange miss on cylinder #6. I've replaced plugs, wires, cap, rotor, ignition module, (it has HEI), adjusted the valves, checked for vacuum leaks, and played with the timing, and NOTHING seems to help. It is a very consistent miss, but only misses below ~1400 rpm, after that it runs perfectly. The engine is a TBI 5.7 block bored to 355, vortec heads with beehive springs, comp xe268 flat tappet, with an edelbrock performer rpm and holley vac. secondary 750. Any ideas on what might be causing this would be much appreciated.