[Elecraft] K3 Audio/RFI Feedback

[Bud Governale, W3LL]

I'm experiencing audio/RFI feedback on 80M between the frequencies of 3650 KHz to 3850 KHz using a full-size 80M 
vertical 100' from the radio.
SWR is under 2:1. No difference if I use the built-in ATU.

If I key the K3 and tap the mic once it goes into motorboating with full output (100W).

The K3 monitor is set at 0.

If I mute the PC line-in sound card there is no motorboating feedback.
If I reduce the K3 mic setting from 30 toward 0 there is no motorboating feedback.
If I reduce the PC line-in volume setting toward 0 there is no motorboating feedback.
If I pull out the cable to the PC line-In there is no motorboating feedback.
If I pull out the cable to the PC line-Out there is no motorboating feedback.
At 50W setting the motorboating starts but is not self sustaining.
At 90W setting the motorboating is self sustaining.

Any suggestions on where next to look - or a solution?

73,

Bud W3LL
w3ll at arrl.net