On the top of the trans, towards the rear you'll see a black box with about 8 wires going in. This is the Transmission Range Sensor. Car off, disconnect it. Start the car and shift a gear. Does the car still die?
If it doesn't, then there is either a broken wire in that group or the range sensor is damaged.
Other than that all I can think of is that the engine is moving too much and putting strain on wiring somewhere, loosing an important connection.
I'd only think the converter if the car had a radiator replaced at some point ... there is a somewhat common problem of the cores cracking and coolant mixing with trans fluid. If not flushed completely there is a risk of early failure of a trans.
Um...You do realize that this is a Ford Focus forum, right? And you are posting in the new members introduction section as well. So I am not sure you are even in the right section, let alone the right forum.
Also, I am moving this to the "General Technical Chat" since it doesn't even qualify as a real introduction.