2000 suburban 1500 RWD transmission going to neutral

so from a cold start it works perfect no lose of
power no strangeness then few minutes later the
engine chokes down for a second and there is a
rattling grinding sound sonewhere around the
torque converter area and it kicks to neutral if i turn
the car off and restart it it goes back in gear drives
fine for a few minutes then back to neutral also if i
wiggle the hell out of the shifter in drive 3rd 2rd or
1st it will choke the motor for a second and
rattle/grind then pop into gear for a short time then
go back to neutral the motor choking then a
grind/rattle is also occurring when its in park idling  
i am no transmission expert by far but to me this
seems like an electrical or maybe ECM/TCM also
my windows dont seem to want to roll up my
hazard lights flash sporadically the volt meter is
not jumping or spiking although i had an issuse
with that last year changed the alternator and it
seemed to have fixed the voltage issuse does
anyone know what would cause this or could bad
voltage to the range switch? Also there is a small
tranmission fuild leak from the shaft output seal  
but i put more then enough fuild in it which i know
bad but dont think it would cause all this unless
theres a censor somewhere im missing but theres
no check engine lights on so im lost