It turns out the problem was temperature related.

I did some digging on this newsgroup and spent a few days thinking
about it, and decided that it might be temperature related considering
I noticed the problem the most on a really cold day. I did a little
experimenting doing about 70mph down the highway with the temp control.
On hot (heater core valve open) the TC unlocked every time. If I
flipped it to cold (valve closed) it would usually lock up within 1-2
miles. I did this 5 or 6 times to confirm, and in every case it did
exactly the same thing.

I bought and installed a new thermostat from a dealer, and the problem
seems to have gone away. I've done one road test so far with excellent
results. If anything, it locks up better than ever. The old
thermostat looked different; I couldn't tell if it was aftermarket or
not. Both were 78C thermostats. I know the last owner had most
everything done at a local mechanic, so it's probably safe to assume it
was an aftermarket part.



wrote:
> I have a similar problem, but in cold weather. I'm certain my
> thermostat isn't closing all the way. When the temperature is below
45
> the engine doesn't warm up enough for the LTC to engage.
>
> Also, the LTC engaging and disengaging was bugging me shortly after I
> purchased the car and was able to fix the problem by adjusting a
cable
> that runs from the throttle to the transmission by the radiator. It
> should be tightened so the slack is taken out of the cable with the
> throttle closed, but no more. Play around with it a bit and you'll
> probably get what you want.