Hey there Scotty, I've got an older 90s 240sx, 130,000 miles all original! (trying to keep it that way) When I purchased it, the auto trans would sometimes pop itself out of gear and stay in neutral or hold first. I did a flush and replaced the filter and gasket, and now It only ever does it in the morning, or if I dont drive it for long periods. When it happens it will either, hold a gear then quickly shift threw all the gears at once, or skip a gear. (Theres no harsh shifts or slipping of gears)  When the car is warmed up or when the sun is out, It will shift normal threw all gears smoothly, without slippage and not give me any issues and act as if nothing was wrong.

What I'm wondering is; Would this be a solenoid or low fluid problem? As It only happens in the morning? Or is the transmission on its way out. Ive had cars with faulty transmissions that had clear signs of failure, this one once I changed the filter and fluid the original *constant* issue went away. Auto transmissions are pretty cheap for these vehicles but would like to know before I buy one and its just solenoid or fluid problem

Thanks again!