Odd KNS-80 Problem, looking for lore-based solution.

Ok, you avionics wonks, here's one for you. KNS-80 problem...

Around the time of installing the GPS, the groundspeed on the
KNS-80 began reading 00 KTS under all conditions while flying
BUT when pinged on the ground--in the aircraft--immediately reads
the correct value even with the test signal turned so low that the
DME can barely lock on.

This has happened in two different late sixties Bonanzas;
one 28v one 14v.

The 14v model had a KLN-90B installed, the 28v a 530W.
Both airplanes have TCADs, both have the KNS-80 talk-only
suppression line properly connected.

The DMEs both have great range, often seeing 80-100 mile locks
below 10,000 feet altitude MSL.

Things tried in flight that have no effect:

1. Stack of 4 KNS-80s rotated into the two airplanes: all
have same symptom. BTW, RNAV works on all.

2. Tune in VOR-DME stations in various directions, various
distances, various frequencies -- all lock but display 00KTS gs.
No Y channels 40us pings (.05 channels) tested.

3. RNAV tried, GS still 00KTS

4. All other radios turned off; no effect

5. All other radios REMOVED from stack... still 00 KTS

6. All other electrical loads shut off; Alternator shut off;
still 00 KTS

7. Various rpm and attitudes.

8. Can't listen to the DME audio in either installation.

Yet to try: Run the airplane while being pinged on the ground.
Expected result: Will indicate correctly.

The DME portion of the box has A+, an antenna, and the
suppression line coming into the box. How complicated
can this be? The DME sends serial distance pulses
internally to
the main 6800 processor (running at 0.95 MHz) where gs
is computed along with the trig for RNAV. (There are a lot
of external lines related to the VOR).

So what the heck can it be? Get rid of the Bonanza?

My plane is the 14v one; I've chosen to ignore the problem for
the last 5 years; the 28v one is being worked on by a local
shop whose customer is far more anal than I and wants it
fixed.

I called King and they are clueless. This one will take
a lore-based solution. I've written it up in detail, investigated
in detail but am missing something.

Am I being persecuted by MI5?

Bill Hale, Loveland CO