I am having trouble keeping my van on while i drive and now in driving position. Please help me?

I have done a little research
and found no solution. My van
started to stall while i drove it
between 5-30 mph now and
then (I took it to a service
department and they could
not find out what the problem
could be at this time); after a
couple of weeks then it just
wouldn't drive anymore (I am
trying to prevent making a
huge payment on it to get it
fixed and i would rather do it
myself if i can.) I am able to
start it then run it in park,
mutual and reverse but when i
place the automatic stick into
drive the engine shuts off
completely. I changed the
"Throttle Position Sensor".
And I put in drive and it iddled
then i shut it off and started it
up again but when i put it into
drive position it didn't
work/iddle again and just shut
down again and again. Please
give me any advice that can
help my situation?