You need to get it to duplicate and have it diagnosed properly instead of throwing parts at it. The ignition contact plate and the gear position sensor were all solid guesses and probably would've worked on 90% of the 850 out there with those symptoms but unfortunately it did not work for you. It could be something as simple as a starter but without diagnosing it this is just a guess. Do you have access to a multi meter and know how to use it and read a wiring schematic because that is what you need to do. You need to get it to duplicate and go out under the hood and see if you are getting 12v to the little wire that goes to the solenoid on the top of the starter under the intake manifold. If you are then you more than likely need a starter if not then you need to see where that wire gets it's feed and trace it back up the line. I know this is probably not what you want to hear but it is pretty much the reality of the situation. You need to get this problem properly diagnosed and then get it fixed right.

Mark