Sometime on its own automatically shifts out of the OD into drive .

It could be because of the brake light switch being activated (shifting it out of OD) by either a fault in it, it's adjusted too close to the top of the pedal stroke, or you resting your left foot on the brake pedal while you drive. I'm not accusing you, lots of nice folks do it. Causes lots of wierd problems. You could check to see that the brake pedal moves down a little before turning on the brake lights, that checks the adjustment. There could also be an electrical problem in the transmission control, internal solenoids, or the wiring to the transmission. If you need more help, try a Toyota specialist or a transmission shop. Here's a directory for you: https://repairpal.com/toyota-repair-in-hotevilla-arizona