Missing does sound like a spark related problem like ben2go said. However, I wouldn't attribute jumping out of gear to low oil level in the lower unit, or bad bearings. Were that the case, it would probably seize up the lower end, not make it jump out of gear. Jumping out of gear would likely be the clutch dog, or the drive gears (presumably the forward gear, unless you are only having this problem in reverse). The way the Johnnyrude lower units work, is there is a pinion gear on the bottom of the driveshaft. This gear will turn at whatever speed the engine is turning at. Then, you have 2 gears meshed into this pinion gear, one in front, and one in the back. However, these gears are in no way affixed to the prop shaft. In fact, there is a bronze sleeve bearing between these, and the propshaft. Both of them spin at the same speed, but in opposite directions, whenever the motor is running. Now, in the area between the gears, the prop shaft is splined, and on these splines is a clutch dog, and there is a detent ball that keeps it in neutral, until you shift gears. On each end of the clutch dog, there are two ears that will correspond with another 2 ears on the drive gears, both forward and reverse. When you shift gears, the shift rod will jam the clutch dog into either forward and reverse, and the spinning ears on the drive gears will mesh with the spinning ears on the clutch dog, and spin the prop shaft. That is why it is good to jam the motor into gear quick, rather than ease it into gear. These ears don't need to slide upon each other, they just need to lock together quickly. If it is jumping out of gear, likely these ears are worn out, due to rubbing together when put into gear slowly, or for other reasons. But, sometimes it is the gears themselves that are damaged, and this is usually due to corrosion eating away at the gears, if they didn't have enough oil, or if water got into the lower end.
Lemme know if this is all jiberish, and I can probably further explain.
