[Elecraft] KPA500 RFI problem

[Ron W3ZV]

At least I'm pretty sure its an RFI problem. I have a K3, KAT500 and the KPA500 connected as recommended with Elecraft DB15HD cables. The antenna is a 120ft. doublet fed with ladder line to a 4:1 balun and 10ft of coax to the KAT500. I have a comment point ground to a ground rod six foot from the shack. I have operated the amp at 400+ watts from 80m to 6m. The only band that gives me a problem is 12m. When I am operating at 400 watts ssb, the amp will fault with a high swr indication within 5 to 20 seconds. Immediately prior to the fault the swr on the amp display was 1.2:1. Reducing power to below 300 watts seems to avoid the problem.

Assuming the consensus is that this is RFI, what would be the most likely suspect? Coax cables in/out? DB15HD cable? Power cable? What's the easiest way to diagnose and correct the problem? Thanks

Ron W3ZV