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.