What Would Cause a Manual Transmission to Pop Out of Gear?by Sam Grover
A problem with some manual transmission cars is that they can pop out of gear. The vehicle will simply drop into neutral unexpectedly, causing the engine to rev without actually turning your wheels at all.
Over time, transmission mounts can become worn. This makes the entire transmission vibrate as the car moves over bumps and uneven surfaces, which can disconnect the engine from the transmission, thus putting you out of gear. The way to fix this is to replace the mounting.
The linkage rods themselves can also start to come loose over time. These are the rods that connect the gear shifter to the transmission. If they are loose, the gear shifter will not stay locked into whichever gear you choose. The way to fix this is to replace the linkage rods.
The bearings that keep the gears pressed together can become worn. As this happens, they stop pressing the gears as efficiently as before, which means the gears can slip apart more easily -- thus dropping your car into neutral. Again, the way to fix this is to replace the bearings.