[TowerTalk] Strange HV-18T HyTower issue

[Bruce Beard]

The Antenna
	   3 year old Hy Gain Hy Tower. Optional MK-17, 17M stub installed, MK160A,160M long wire installed .
DX Engineering radial plate.
Fed with Andrew Heliax hardline. 
4000+  feet of radials. 

The Installation:
	Mounted in concrete. Located in heavily wooded area.  lots of leaves and debris, constantly cleaning stuff from base of antenna, never seems to make any difference.

The problem:
	First noticed that while working on 75M and 160M, after transmitting, the antenna would go quiet. It is as if the attenuator is coming on or the antenna is being disconnected. If I put just a small amount of RF into the antenna system it "wakes up". This condition is more apparent on 160M than 75M.
SWR is fine under power;  however, I have noticed that SWR (indicated on my LP 100A) sometimes begins rising, in ssb while keyed but without drive (i.e. keyed but not speaking). Speaking and thus putting power into the system brings SWR back down.

Now I believe I am getting a flashover somewhere. Plate voltage will spike on the amp after transmitting with power thus shutting off the amp. My OCF dipole is fine with the same radio/amp/tuner combination. My Yagi is fine with no symptoms although I don't use the tuner for the Yagi.

 Replaced coax last year with no apparent change. Dropped antenna for visual inspection and saw nothing that looked unusual.

I am at a bit of a loss here and I need suggestions  as to where my problem might be or how to diagnose this (these) issue (s).
 
73,
Bruce Beard
AB1KC
ab1kc at arrl.net