I have a 1998 Honda Odyssey that I am the 2nd owner on.  Bought it 3
years ago with 80K miles and am at 120K now.

The car has run great for me though last year a problem arose which is
unsettling and a bit of a nuisance but otherwise something I would
rather live with than follow the dealer's advice (more on that later).

The problem is that occasionally (and more frequently recently) the
computer throws a Torque Converter code and the transmission drops
into "limp home" mode which feels like 4th or 5th gear.  When it does
this I can shift down to 1st gear to get up to speed from a start
after which I put it back to D3 or D4.  90% of the time I can pull
over, turn the ignition off and re-start the car and I'm back in
business with a fully working transmission.  THe other 10% of the time
it either goes right back to limp home or sometime shortly after
driving away the problem recurs.

I know my gas mileage is about 15% worse when I drive it with the
problem, and when I go over 70 mph the car seems to lurch/hesitate,
but am I hurting the car by driving it when it is in this "limp home"
mode?  Does it make a difference if I'm driving it mostly at highway
speed?

Now for slightly more (essential) details and a followup question: 
the computer has also thrown another code, not always at the same time
as the Torque Converter code but I think most of the time and seems to
me to be related: the code it's throwing is  P1607 (PCM Failure). 
THat makes me wonder whether in fact I even have a Tranny problem.  My
salt of the earth mechanic said he doesn't want to touch it with a
potential tranny issue and his tranny guy said he doesn't want to
touch it with the PCM problem since he says only Honday holds the keys
to that kingdom.  When I finally caved and brought it to the Honda
dealer they said that the only code they read was the PCM failure code
and given my explanation thought that if I was unwilling to replace
the PCM (which is what their manual told them to do for the PCM code)
was to sell me an overpriced tranny flush which of course did nothing.
They were adamant that selling me a new PCM would not be guaranteed to
fix the problem.  What would you recommend?  FWIW, the problem does
seem to be occurring more frequently, though it always has seemed to
be worse in the colder seasons.  When I go on a long road trip I
typically see the problem for the first 50-100 miles and then re-start
the car and can go a full day of driving without seeing it come back. 
Of course it's right back there when I start the next morning. 
Advice??