Code P768

[TLDG]

I have a 2000 Lexus ES300 with 143,000 miles on the clock. For the past couple of years the engine check light would come on with a P768 code, which indices the transmission is in the wrong gear for the current speed.  It is generally diagnosed as a faulty "D" solenoid in the transmission, but in my case I do not think that is true.  I bought a diagnostic tool that plugs in under the steering column and lets me reset / erase the code and turn the check engine light off.  However, upon restarting the engine, and with the car absolutely stationary, upon moving the gear selector from Park to Reverse (or any other position) the light comes back on (sometimes).  I have driven the car across the country with the light remaining off for hundreds, even thousands of miles, only to have it come on after a fuel stop.  It seems totally random, and the transmission actually shifts correctly.  Also the problem does not seem to occur until the engine temperature has warmed up at least slightly.  Even with aroung the town driving or short trips to the post office or supermarket, the light may remain off for days or weeks and at other times will come on repeatedly.  Has anyone else encountered a similar problem?  I am inclined to think there is just a loose wiring connection somewhere but at current labor costs a techician might spend several hours searching for the problem and never find it.  I would appreciate any insight from others who have experienced this problem and what was done to correct it.

[Lexus Owners Club]

I would start with checking fluid level and condition in the Auto Transmission.
Change if necessary and also the filter at the same time.

Whilst your under the car then it is also worth checking the electrical conditions for security