What can I do about oil pouring out of my 2002 Buick Rendevous with 23,000 original miles?

I bought car from friend 1 week ago.  I live in mountains, and I drove it down the hill, all
over San Bernardino, and back up the hill with no problems whatsoever.  Next day, I
started it up and drove 10 ft. and all of my oil spilled out of engine.  My friend looked at
it and said oil was coming out of oil sensor and we replaced it.  Then we looked in
radiator and cap had that oil/water goop stuff on it.  No overheating or smoke the day
before or this day.  We flushed it out, changed oil and filter and radiator fluid, started it
up and it was fine for about 1 minute.  Then all the oil blew out all over the ground.  The
next day, we looked at the oil filter and the seal was sticking out a little bit.  So we
replaced the seal, put in more oil, turned car on and immediately oil blew out all over
the ground again.  I read a similar story and did what the guy said to do and that was to
change the oil filter again in case it was put on wrong or faulty.  I did that yesterday,
and we turned it on and no oil leak whatsoever.  I drove it about 5 miles and about 2
miles the check engine light came on.  At first when I drove it, for a few minutes it said
to change the oil, but eventually it went off.  When I got home, my car stalled when I
was getting ready to back in the driveway.  I noticed oil in the street again.  Put more
oil in so I could watch what it was doing, turned it on and all the oil came out again.  
Help!