The reason the gear lever becomes stiff and jams is because the shaft on top of the transmission casing has ceased up. When it does this you need to remove the battery and battery tray. You will then see the shaft with its quadrant case. Take off the top case so you have the shaft revealed then spray with WD40 and gently rock the shaft with a spanner from clockwise and anti clockwise. Spray more until it’s freed. Do this for around 5 mins to work in the lubricant then add some grease on top. Re-assemble then check the gear selector is back to normal. If ok add the battery tray and battery back in.