Second what coilover said. Lift, don't push, the shifter from first to neutral, let it drop into the middle, then push it up into second. I had the exact same locking up problem on mine until I learned to shift correctly.
Popping out of second sounds like a worn shift fork. Pull the side cover and check the interior surface of the fork for a deep wear groove. If it is, then replace. If you continue to drive it with it popping out of gear constantly the square ends of the gear teeth will wear on an angle and eventually you won't be able to hold it in gear no matter what. This is the same problem that killed my first transmission.
