car seems to slip out of gear

The car will randomly go out of gear. I can be driving and all of a suddden it just revs up and doesnt accelerate. Sometimes i can either put it in neutral and turn it off and then back on and then when i put it in drive it will be in gear or i can stop put it in park turn it off and then when i turn it on it is ok. But here lately it is doing it more frequently and seems to be more when it is extremely hot outside. Also i can tell if it will shift or not by how it feels after i put it in drive. For example if it has happened and i stop and turn the car off when i take it out of park and put it into drive it will jerk slightly so i know it isnt going to shift but if it makes no jerk it will shift...but sometimes it will go into drive smoothly and then when i take my foot off the brake and put it on the gas it will jerk and then not shiff...or today i was stopped by a train and i put the car in park while i was waiting and after about 3 minutes it jerked while in park and then wouldnt shift when i started driving. It also sometimes when i am braking to slow down the car will jerk then too going out of gear. It doesnt happen all the time and there are no codes coming up besides lean fduel so any help would be greatly appreciated. I was told by someone it may be the speed sensors but i changed both of those and no luck. Thanks so much.