PA28-181 - Stuck Mike - King Silver Crown

My Archer has the original avionic stack, KMA20, KX170B, KX170B etc.
Single PTT on Left Yoke.

A couple of weeks ago the mike stuck open at the end of a 2 hour flight.
The following morning I checked the system and found that I still had a
"stuck mike".
I assumed that it would be the PTT switch which I removed and checked - no
shorting.

I then made the following checks.
1. Switched off both comms and navs
2. With Com 1 selected in the KMA20, I powered up Com 1 - Continuous
transmission - Powered Com 1 down.
3. With Com 2 selected in the KMA20, I powered up Com 2 - Continuous
transmission.
4. Selected Com 1 in the KMA20, powered up Com 1 and proceeded to work all
the front switches on the KMA20.
When "exercising" the Marker beacon switches, the stuck mike condition
unstuck itself for a few seconds then returned.
Further toggling of the Marker beacon switches finally unstuck the
condition.
5. Flew the aircraft for an hour and stuck mike returned.

In the air with the mike stuck open, there is a tremendous fast pulsing
sound coming from the aircraft speaker, although the speaker was not
selected by any switch on the KMA20.

I concluded that the problem lies in the KMA20.
I sort advice from 2 local avionic shops who both thought the problem would
not be in the KMA20.
The problem now re-occurs after about an hour in the air but after a spell
on the ground goes away.
It must be heat and or vibration related.
Yesterday the shop spent several hours and declared it was the PTT switch,
fitted a new one and said I was good to go.
On the way back to base, guess what, stuck mike again.
Sat on the ground for an hour - no stuck mike.
Called the shop - flew it back - more hours - could not find the problem -
unable to replicate the stuck mike problem running engine on the ground.

Advice and help please because I reckon no radio shop is going to be able to
cure the problem until the failure becomes permanent.
Thanks

Roy