Intermittent slipping out of gear

My car randomly slips out of gear into neutral while driving from once every other day to three times per day. It slips out of gear from five to twenty seconds and then pops back into gear and if not letting up on the gas, it lurches forward when it pops back in. Given the intermittent nature, I don't thinks it is a mechanical problem related to the transmission. I think it must be electronic related to either the neutral safety switch, a bad solenoid or maybe the ECM/ECU. I have taken it to the dealer and they could not reproduce it and recommended I replace the whole transmission at a hefty price tag. I took it to a recommended private repair shop and they charged me $135.00 and gave it back to me and said nothing is wrong with the car but still charged me. It actually is quite dangerous, not sure what to do.