Sounds like engine control/electrical problems, possible crank sensor problem. Key in on the tach dropping to zero and the engine losing power (this is ignition related). Fix this first. A bad range switch can cause random shifts, but from your description it doesn�t sound like that is what is you have. Also the VSS and TSS sensors can cause erratic shifting, but again, fix the tach problem first as this can also cause bad shifting.
jeff