Last night I tried coiling the mic cable as much as I could without shortening it to the point that it was unusable. I even put some ferrites around the coil. I thought I detected a slight change but the problem was still there.
I also tried the stock mic, the problem did not occur at all. I use a much lower SSB mic gain setting for the Koss headset vs the stock mic as the Koss is much more sensitive. I forgot to change the gain back to what it should be for the stock mic so that could have made a difference. I'll try it again with the stock mic and the normal gain setting that I use for the stock mic.
The Koss head set is made for computer work not RF work and has no RF shielding at all you will end up having to completely re-design the circuitry in the Koss to get it to work properly. I tried to get a Sennhesier Computer Headset($115) to work and I just could not do it. 40M and 15M were affected horribly. I switched to a Heil Proset and never had a problem after that even at over 1KW+. My antenna is a 3element SteppIR with the 30/40 Driven element only 15ft over the shack. The House is on stilts 12ft off the ground and the antenna is on a Glen Martin roof tower.